Ok, so basically the only components that GM actually makes are the frame, body panels, transmission and engine. An assembly plant will always be supplied by another GM transmission and engine plant and some assembly pants have stamping shops that make their own panels, while some don't.
Every other part is a supplier part and there are only so many suppliers. So a supplier that sends GM radios, seats, air conditioners, carpet, instrument panels, spoilers, mirrors, etc. also sends parts to other automobile manufacturers. That being said, there are different levels of these products and a radio in a Cadillac is going to be more expensive than a radio in a Chevy and that is what accounts for the difference in price.
I have been in more OEM plants than I can count and I can tell you that General Motors makes really nice vehicles. I am basing that on the manufacturing processes and quality of components that they use. Surprisingly, I would say that Tesla makes the worst vehicles in terms of these things. They are built with really shoddy parts and their quality numbers reflect this as they are really rated poorly.
I do not have any experience with KIa, but based on the information that I have being in the industry, I would never buy one.
So your point about Honda and Toyota being ahead of GM with electronics is just untrue because the manufacturers of the electronics in all of these vehicles are pretty much the same.
I would be more than happy to answer any questions.