The "domestic content" calculations are based on sourcing of components in the U.S. Everything from the steel sheet, to tires, to electronic components, seats, and engines is broken out according to whether they are sourced in the U.S. or elsewhere. As a practical matter, even the profit of the "foreign" makers in the U.S. generally stays here. It is reported on the books back home, but the actual funds are rarely sent back.
If you, as a consumer, want to maximize your purchasing of U.S. domestic-made goods, these percentages are a fairly reliable guideline to follow. Also, the dealerships and domestic infrastructure take a significant percentage of the purchase price, and that is U.S. "content."
Even the design of most "transplant" cars is done in the U.S., as the manufacturers realize that cars designed in Japan and Korea often lack the styling cues that attract American buyers.
Parenthetically, I have worked for U.S. domestic subsidiaries of foreign companies for most of my working life. My one overt attempt to do otherwise was thwarted when the United States Filter Company was, shortly after they hired me, purchased by Vivendi of France.
