"Japanese car plants are mostly just assembly...The parts mostly come from Japan...As well as replacement parts..."
Don't forget all the tooling, machines to run that tooling, and all the engineering and design work
That is incorrect. I have a "Made in Ohio" Honda that has a higher US part content (80%)than the Ford I owned previously. I also worked (long ago) for a US steel company that produced steel body panels for Toyota, Mitsuibishi and Honda. All the shipments were to the US plants of those companies.