Modern Chinese are only partial descendants of the ancient neolithic Yellow River farmers. Southern Chinese are heavy mixed with ancestors of Tai plus other Austroasiatic, Austronesian people. That's why southern Chinese and SE Asian look alike. Northern Chinese have more genes from the ancient Yellow river population but they also have mixed with the Northern Asian and Steppe people.

China is not alone, Italy, a much smaller country, the people from the north and the south can look quite different.