Note that the allele frequency is generally zero in African populations, and is over 90% in the Han. It’s also pretty common in pure-blooded Amerindians, which means it’s fairly old, since they came over from Asia 15-20 k years ago. It’s been favored by selection, in some unclear way, which can easily drive an allele from one copy to the high frequencies we see today in China, given maybe 30,000 years.
Most alleles don’t vary enormously in frequency between different human populations, because those populations have a common origin – maybe 70,000 years ago for people outside of sub-Saharan Africa, maybe 150-250 k years ago for the various groups in sub-Saharan Africa. But this one does, and there are others, like DARC ( near 100% in Africa, unknown in China).
You can find populations all the way into Europe ( east and north) and India that have low, non-zero levels of EDAR370a. So there was some gene flow between such groups over the past few tens of thousands of years, but not a lot. How much gene flow was there between more distant groups ( over the past thirty thousand years or so) , say between China and the Khoisan? Or between China and Spain, China and England, China and Italy?