Americans used to be predominantly white, but now there are many blacks and many mexicans and asians who have US citizenship. They may be naturalized or they may have gotten the citizenship at birth. A lot of Americans who are NOT your standard white American, like to add that they are Korean-American, Chinese-American, etc.
Having a passport does not automatically mean you define yourself as being from that country.
I am born in Taiwan , for example, to an American father and a Taiwanese mother. I got US citizenship by virtue of at least one of my parents being American (which in my case was my father). I was raised on Taiwan and tend to identify myself as Taiwanese, rather then American (although my citizenship is American).
I am a true Taiwanese-American , I could say.
Why do Chinese like to identify themselves as Chinese? Because the Chinese diaspora around the world do this. IF you are half Chinese or full Chinese, most Chinese will consider you Chinese. If you have no Chinese blood , then you won’t be considered Chinese. I guess that , those of us who are , at least in part Chinese like to say we are to include ourselves in the Chinese diaspora. Unless we have no association at all socially with any Chinese people. IN which case it may be less important.
A Chinese person will accept another one as “Chinese” if they are so as a matter of fact. This way you don’t get treated as “foreign”.
Although being only partially Chinese (or more correctly TAiwanese) I am never made to forget that I am half something else though by others who are “full” Chinese. And one must remember that most Taiwanese are in part TAiwan aborigine.
But Taiwan is changing. And in future a lot of “Taiwanese” will have foreign blood. Many vietnamese wives here, also from Thailand and also many western spouses now. So Taiwan is changing. Being Taiwanese will no longer mean mainly Chinese even.
There will also be cases no doubt where someone is going to be born in Taiwan of white parents (or another set of non-chinese parents) but be a citizen. He/she is going to be Taiwanese too.