When we talk about Chinese people, everyone knows who were are talking about. It is the common person. The Han. The other 55 are just groups who happen to have been in the territory China controlled at a particular date who had to be incorporated into the state to avoid revolts. Even then some (but not all) incarnations of the Chinese state would just try to eliminate them if they rebelled because that was viewed as if it would be easier in the long run than dealing with constant revolts in territory that was now deemed to be part of the "China" now.

Don't take this as a claim that china is this innately xenophobic society but they tend to oscillate between periods of extreme openness and extreme closedness, and the "bad" "prior" China might do something that eliminate a group and turns them into Han, and the "new" "good" China might condemn the prior China as having lost the mandate of heaven due to their crimes but the long term trend of these is that there is an ever expanding block of people who are regarded as Han, and it is that expanding block that people consider to be Chinese. Of course some of the old ways get preserved despite these state actions so the regional variation of the Han is extensive even if they are all regarded as Han (not all Han even speak in ways that are understandable to each other, although the Chinese state does not regard these as languages and instead calls them dialects. The Chinese dialects are however all derived from each other and fit into a language family, so it is a bit like with Dutch and English where you can tell they are kind of similar even if they are not understandable to one another and the Dutch have an easy time of learning English due to the similarities).