Many "American" cars have been made in Canada since the beginning. In the beginning US automakers set up factories in Canada for export to Britain to take advantage of Canada being duty free as part of the empire. Then in 1965 the Auto Pact was signed which unified the US/Canada car market in exchange for American car factories in Canada. Since then Canada has produced ~10% of "American" cars, but has imported a similar number of American cars.
NAFTA/USMCA allows some manufacturing in Mexico/Canada (Which has largely equivalent labor costs), but if these manufacturers pulled out of the US, it would be repealed before you could blink.