You don't have to talk about Cantonese and Fujianese like all other Chinese have to stay away from them. There certainly is higher concentration of other countries intermixing with these areas of trade. There is nothing wrong with trade. But technically speaking that is all the Chinese eastern seaboard from Beijing to Hainan because of the simple waterway called the ocean. However people found very into the interior are also intermixed. From history we can tell people from the former Xi Xia area were Tanguts and the people must have found there way into the interior of China starting with Shanxi and Shaanxi.
Hanized natives...hanized outsiders, all hanized by the various dynasties that ultimately taught themselves to use the chopstick.
Don't forget there are also other dynasties, by different names, originating from the Central Plains, Mongols and Manchus. However, My conclusion is that China are people of different genes, some more alike than others and the similar ones were group as Hànrén by the new post imperial leaders, for nation building purposes. Others are obviously distinct races, like the Tibetans, Uighurs, Mongols, Manchus, Zhuàng, etc and that was why they were independantly grouped.
People from Sìchuān are originally Tibetans and that is why Tibet included it in their claim. That may expalin the high cheek bones. Most people under the subset Hànrén seems to have emerged as one 'race' though it is confusing to many that they can look very different.
The best way is to see it like USA. The Americans comprised of many races who migrated there but they are all called Americans. Chinese is not a race but a name for all nationals of China.