I think your claim that China and the US are the biggest markets due to being homogeneous is flawed. The United States is far from homogenous; in fact, it is on the way to becoming a minority majority country. This is quite different from China’s demographic makeup.
The relevant homogoneity isn't ethnicity, it's culture. Every American knows to
look for white signs on the road for speed limits, to raise your hand if you
have something to say in a group, and that Football is played on a field 100
yards long. These shared folkways ease the function of social life and enable
commerce. You could pluck someone off the street of Montanna, drop them in New
York, Louisiana, or even Guam and they could easily navigate, make purchases,
share not only a language but likely interests and pastimes with locals, and
order their favorite meal from a national chain restaurant.
I think your claim that China and the US are the biggest markets due to being homogeneous is flawed. The United States is far from homogenous; in fact, it is on the way to becoming a minority majority country. This is quite different from China’s demographic makeup.