I wonder if China’s population is more or less ethnically and culturally homogeneous than that of the USA, and whether this should be considered a bit of an apples to oranges comparison.

the defining characteristic(s) of an ethnic minority is a artificial distinction that the culture you exist within conditions you to regard as important and it will be different from one culture to the next.

thanks to colonialism, some of it will have a superficial analogue to your own culture, but the ingrained distinctions stretch back for hundreds to thousands of years longer and most are imperceptible to foreigners; hence the entire "monoculture" assumption by westerners of the (mostly) non-melinated variety.