The very reason why people "by" from China is that it's cheaper. Buying from the US might cost even more than from China with the Tariffs in place.
When people think of China, they think of cheap plastic dollar store toys and kitchen utensils.
They don't think of all the electronic components, metal/steel components, vehicle components, raw materials that can't be so easily sourced from other parts of the world, etc etc.
Trump really has no clue the long line of dominoes in play when it comes to (specifically) the various manufacturing industries.
He has no clue how much it can cost a company for every hour they might have to stop their assembly lines because they don't have the parts they need to manufacture their finish product, or the parts they've received are inferior.
As an example: When you're in the business of making transit buses, you can't just buy your chassis from any ol' supplier anywhere in the world. That chassis not only has to pass welding xrays and a dozen other engineering quality inspections, it also has to be made to the 1/1000th of a millimeter to your specs in order for it to fit all the other parts you have coming in from other suppliers around the world. It's a whole bunch of fine-tuned calibrated puzzle pieces that all have to fit together perfectly to make the end product (the transit bus).
Depending on the type of manufacturing industry you're in, the design schematics of your particular product and the specifications of the parts you need to slap it all together can be very detailed and customized.... a long process of design configurations and reconfigurations between you and your supplier(s) before their manufactured component meets your needs.
But according to Trump, it's just a simple matter of picking up a phone and finding a new supplier. Easy peasy.
The guy is a ******* moron.
Edit to add: And the point you make about the cost of parts/components/materials also plays an important role for these manufacturing companies. Maybe they'll be able to source their components from some company in the US, but at a much higher price... which of course trickles down to the consumer. Chances are they'll just continue to buy their products from China because it will still be cheaper.
The bottom line is, everyone will feel the pain of Trump economics. From the raw material supplier, to the component manufacturer, to the shipper, to the end product manufacturer, to the store, to the consumer. All the way down the line.