Indeed. HDenier keeps repeating the same misunderstanding of how we know about the Holocaust - in fact, about any historical event or period (because the methods we use to investigate and interpret history do not change because of the object of study): it is not because of a single piece or kind of evidence that we can draw conclusions about the past but on account of a range of pieces of evidence of different types that when taken together create a picture of what happened.
In one sense what we have to explain is all that evidence: We have photographs of the camps, records of where many of the transports went (Gedob records and surviving Fahrplanordnung, bureaucratic correspondence) and other railway timetables, evacuation orders, inventories of goods stolen from victims, construction records (and Pressac's 39 criminal traces, e.g.), etc, various kinds of statistical records and reports, archaeological and other physical evidence at the camps themselves. Further, we have some interesting things like reports from killing squads themselves as well as from the Wehrmacht - some of these, the Ereignismeldungen, were used to convict Einsatzgruppen leaders of murder without witness testimony. We've got documents related to the use of gas vans (e.g., 501-PS and others). We have planning documents and discussion records; we have SS internal investigations and court decisions; we have military war diaries and diaries kept by Frank and Goebbels; we have decrees, orders, and other records relating to implementation of the Final Solution. We've got personnel records of the SS, other police organizations, and Wehrmacht, we've got official reports on the results of actions in making areas free of Jews. And so on.
And we have other documents - newspapers, records of parliamentary debates, speeches by political leaders, and committee hearings. We have diaries and letters written by victims and observers. we have a wide variety of photographs and newsreels and other film footage, we have forensic and medical information (physical studies use increasingly sophisticated investigatory and modeling techniques), we have geographic data and geographic modeling methodologies, we have census records, we have business documents, government decrees and ordinances, internal memoranda, meeting protocols, audio recordings, orders, telegrams, log books and ledgers, architectural blueprints, forensic reports and so on. Yes, we also have the one type of evidence HDenier goes on about, and misinterprets - memoirs, witness testimony, and oral histories.
Using all these kinds of evidence, scholars work out what happened, how, and what people experienced during the period. Unlike HDenier, they do not look at a single kind of evidence in isolation. Given the complexities of evaluating different kinds of evidence, much of it complex, and their interrelationships, historians debate how to best interpret it. Real revisionism is the use of evidence to come to new understandings - not mindless repetition of a few catch phrases or supposed debunking exercises. Real revisionists do not hop around from slogan to slogan but sort through previous interpretations and old and new evidence to re-evaluate historical topics. Also, it has to be noted that historians routinely, as part of their craft, subject all evidence to scrutiny to weigh its value and determine what it can tell them - HDenier's implication that scholars stupidly accept on its face values whatever they come across shows how little HDenier knows about how historians work.
The pointlessness of the 'one source at a time' approach is glaringly obvious to anyone who looks down to the footnotes or flicks back to the endnotes of a history book. It's even clearer when one looks at more than one book.
References generally point to sources - these might be other books and articles which in turn cite their sources, or these might be primary sources of the various kinds LemmyCaution outlined. Historians working in a particular field will have many sources in common, but they also discover new ones.
Here's a survey of 148 books, 97 of which are in English, about different aspects of the Holocaust, Nazi crimes, and their aftermaths, a list that hardly even scratches the surface of what has been published on these subjects, but which includes many works that are generally regarded as essential.
They used a total of 221,641 endnotes/footnotes.
Even with repeated citations of the same core sources (such as the Wannsee protocol) and secondary literature across many different books, the number of relevant sources runs into the many 10s of 1000s.
Poliakov, Harvest of Hate 1951 481 notes
Reitlinger, The Final Solution 1953 1660 notes
Hilberg, Destruction (1961 1st edition) 1961 3413 notes
Levin, The Holocaust 1970 693 notes
Dawidowicz, War Against the Jews 1975 598 notes
Hilberg, Destruction (1985 2nd edition) 1985 4329 notes
Gilbert, The Holocaust 1986 2097 notes
Yahil, The Holocaust 1990 1784 notes
Friedlander, Years of Persecution 1997 1147 notes
Hilberg, Destruction (3rd edition) 2003 4711 notes
Friedlander, Years of Extermination 2006/7 2431 notes
Bloxham, The Final Solution 2009 557 notes
Longerich, Holocaust 2010 2263 notes
Goda, The Holocaust: Europe, the Jews and the World 2013 1440 notes
Cesarani, Final Solution 2016 1604 notes
Gerlach, The Extermination of the European Jews 2016 2404 notes
(23,870)
Trunk, Judenrat (English) 1972 2029 notes
Kogon et al, Nazi Mass Murder with Poison Gas 1983 551 notes
Benz (ed), Dimension des Völkermordes 1991 2703 notes
Aly, Final Solution 1995 982 notes
Browning, Nazi Policy, Jewish Workers, German Killers 2000 586 notes
Corni, Hitler’s Ghettos 2002 2162 notes
Wildt, An Uncompromising Generation 2002 2389 notes
Browning, Origins of the Final Solution 2004 1853 notes
Stone (ed), The Historiography of the Holocaust 2004 1822 notes
Aly, Hitler’s Beneficiaries 2005 997 notes
Cüppers, Wegbereiter des Shoahs 2005 1719 notes
Westermann, Hitler’s Police Battalions 2005 1166 notes
Klemp, Nicht ermittelt 2005/2011 1497 notes
Gruner, Jewish Forced Labor under the Nazis 2006 1290 notes
Hoffmann, ‘Das kann man nicht erzählen’ (1005) 2008 1135 notes
Steinweis, Kristallnacht 2009 411 notes
Stone, Histories of the Holocaust 2010 926 notes
Morsch/Perz (eds), Neue Studien 2011 1249 notes
Confino, A World Without Jews 2014 418 notes
(25,885
Laqueur, The Terrible Secret 1980 280 notes
Wyman, The Abandonment of the Jews 1984 1281 notes
Favez, The Red Cross and the Holocaust 1988 (1999) 565 notes
Phayer, The Catholic Church and the Holocaust 2000 969 notes
Leff, Buried By The Times 2005 876 notes
Herf, The Jewish Enemy 2006 842 notes
Longerich, ‘Davon haben wir nichts gewusst!’ 2006 1233 notes
Dörner, Die Deutschen und der Holocaust 2007 2265 notes
(8,311)
Bloxham, Genocide on Trial 2001 989 notes
Douglas, The Memory of Judgment 2001 465 notes
Weinke, Die Verfolgung von NS-Tätern im geteilten Deutschland 2002 1499 notes
Weindling, Nazi Medicine and the Nuremberg Trials 2004 2189 notes
Hebert, Hitler’s Generals on Trial 2010 870 notes
Segev, Simon Wiesenthal 2010 1026 notes
Heller, The Nuremberg Military Tribunals 2011 2400 notes
Eichmueller, Keine Generalamnestie 2012 1352 notes
Raim, Justiz zwischen Diktatur und Demokratie 2013 5422 notes
(16, 212)
Burleigh, Death and Deliverance 1994 972 notes
Friedlander, The Origins of Nazi Genocide 1995 1745 notes
Aly, Die Belasteten 2013 454 notes
(3,171)
Streit, Keine Kameraden 1978/1991 1780 notes
Krausnick/Wilhelm, Truppen des Weltanschauungskrieges 1981 2107 notes
Gerlach, Kalkulierte Morde 1999 6309 notes
Dean, Collaboration in the Holocaust 2000 1019 notes
Ioanid, The Holocaust in Romania 2000 1378 notes
Angrick, Besatzungspolitik und Massenmord 2003 1933 notes
Berkhoff, Harvest of Despair 2004 1620 notes
Kruglov, The Losses Suffered by Ukrainian Jews 2005 1508 notes
Lower, Nazi Empire-Building 2005 756 notes
Curilla, Die deutsche Ordnungspolizei und der Holocaust 2006 8270 notes
Angrick/Klein, The “Final Solution” in Riga 2006 1282 notes
Kay, Exploitation, Resettlement, Mass Murder 2006 953 notes
Brandon/Lower (eds), The Shoah in Ukraine 2008 1105 notes
Arad, Holocaust in the Soviet Union 2009 1747 notes
Weiss-Wendt, Murder Without Hatred 2009 1339 notes
Dieckmann, Deutsche Besatzungspolitik in Litauen 2011 6712 notes
Reichelt, Lettland unter deutscher Besatzung 1941-1944 2011 1270 notes
Rentrop, Tatorte der ‘Endloesung’ 2011 969 notes
Rein, The Kings and the Pawns 2011 1472 notes
Ancel, The History of the Holocaust in Romania 2011 1909 notes
Kay, The Making of an SS Killer 2016 896 notes
(46,334)
Gutman, The Jews of Warsaw 1982 698 notes
Krakowski, The War of the Doomed 1984 565 notes
Engel, In the Shadow of Auschwitz 1987 911 notes
Arad, Belzec Sobibor Treblinka 1987 688 notes
Browning, Ordinary Men 1992 511 notes
Engel, Facing a Holocaust 1993 903 notes
Pohl, Von der “Judenpolitik” zum Judenmord 1993 863 notes
Pohl, Nationalsozialistische Judenverfolgung in Ostgalizien 1996 2153 notes
Sandkühler, “Endlösung” in Galizien 1996 1050 notes
Bender, The Jews of Bialystok 1997 (2008) 1018 notes
Schelvis, Sobibor 1998/2003 662 notes
Krakowski, Chelmno 2001 459 notes
Paulsson, Secret City 2002 547 notes
Rossino, Hitler Strikes Poland 2003 1100 notes
Poprzeczny, Hitler’s Man in the East: Odilo Globocnik 2004 463 notes
Crowe, Oskar Schindler 2004 1836 notes
Schwindt, Majdanek 2005 1475 notes
Ziołkowska, Obozy pracy przymusowej 2005 746 notes
Alberti, Verfolgung und Vernichtung 2006 1866 notes
Löw, Juden im Getto Litzmannstadt 2006 1735 notes
Loose, Kredite für NS-Verbrechen 2007 2023 notes
Mlynarczyk, Judenmord in Zentralpolen 2007 1338 notes
Rutherford, Prelude to the Final Solution 2007 744 notes
Rieger, Creator of Nazi Death Camps 2007 551 notes
Kassow, Who Will Write Our History? 2007 1193 notes
Horwitz, Ghettostadt 2008 888 notes
Roth, Herrenmenschen 2009 1233 notes
Klein, Gettoverwaltung Litzmannstadt 2009 1635 notes
Mailander, Female SS Guards and Workaday Violence 2009 (2015) 1408 notes
Puławski, W obliczu Zagłady 2009 2386 notes
Wardzyńska, Był rok 1939 2009 277 notes
Epstein, Model Nazi 2010 1907 notes
Browning, Remembering Survival 2010 1449 notes
Kuwalek, Das Vernichtungslager Belzec 2011/2013 713 notes
Curilla, Der Judenmord in Polen 2011 8405 notes
Montague, Chelmno and the Holocaust 2011 675 notes
Berger, Experten der Vernichtung 2013 1698 notes
Klemp, Vernichtung 2013 590 notes
Rashke, Useful Enemies 2013 542 notes
Silberklang, Gates of Tears 2013 748 notes
Bryant, Eyewitness to Genocide 2014 667 notes
Benz, Handlager der SS 2015 1185 notes
Zimmerman, Polish Underground and the Jews 2015 2067 notes
Hembera, Die Shoah im Distrikt Krakau (Tarnow) 2016 1548 notes
(56,119)
Browning, The Final Solution and the German Foreign Office 1978 811 notes
Safrian, Eichmann’s Men 1993 (2010) 1109 notes
Lozowick, Hitler’s Bureaucrats 2000 (2002) 787 notes
Gerlach/Aly, Das letzte Kapitel 2002 1870 notes
Cesarani, Eichmann 2004 1024 notes
Meyer, Täter im Verhör 2005 1,599 notes
Weitkamp, Braune Diplomaten 2008 2112 notes
Meyer, Das Wissen um Auschwitz 2010 556 notes
Stangneth, Eichmann Before Jerusalem 2011 (2014) 1406 notes
(11,274)
Pressac, Die Krematorien von Auschwitz 1994 318 notes
Gutman/Berenbaum (eds), Anatomy of the Auschwitz 1994 1160 notes
Dlugoborski/Piper (eds), Auschwitz 1940-1945, 5 vols 1995/2000 3286 notes
Dwork/Pelt, Auschwitz 1996 679 notes
Wagner, IG Auschwitz 2000 1308 notes
Steinbacher, ‘Musterstadt’ Auschwitz 2000 1632 notes
Pendas, The Frankfurt Auschwitz Trial 2006 1227 notes
Schüle, Industrie und Holocaust 2010 1301 notes
Fleming, Auschwitz, The Allies and Censorship 2014 847 notes
(11,938)
Orth, Das System der nationalsozialistischen KL 1999 1234 notes
Schulte, Zwangsarbeit und Vernichtung 2001 1778 notes
Allen, The Business of Genocide 2002 726 notes
Strebel, Das KZ Ravensbrück 2003 2625 notes
Buggeln, Arbeit & Gewalt (translated as Slave Labor) 2009 1976 notes
Caplan/Wachsmann (eds), Concentration Camps in Nazi 2010 675 notes
Blatman, The Death Marches 2011 1836 notes
Jardim, The Mauthausen Trial 2012 616 notes
Weindling, Victims and Survivors of Nazi Human Expts 2015 1042 notes
Pauer-Studer/Velleman, Konrad Morgen 2015 510 notes
Wachsmann, KL 2015 3423 notes
Hördler, Ordnung und Inferno 2015 2086 notes
(18,527)
(148 books and collections; 97 in English) 221,641
As I said, this is just a sample of what is out there.