There's a long, long history of China controlling Tibet, back at least to the Tang dynasty. The ChiComs are nastier about it than most of the previous Chinese governments, but that's what you get when you refute the old mandate of heaven/chakravartin mashup of political legitimacy espoused by the old empire for ... what justification do the ChiComs give for their political legitimacy? Having the biggest boot and the willingness to murder millions and imprison more, I suppose.
Well, it's right there in art. I of their own Constitution, I think: "The People’s Republic of China is a socialist state under the people’s democratic dictatorship led by the working class and based on the alliance of workers and peasants."
It might have its problems (will the country forever have "workers" and "peasants"? Does the working class include peasants? Who does a democratic dictatorship actually dictate to? etc etc) but it's still better than divine mandate, tbh, which might explain why it's not been toppled yet.
Or the Pacific islands or South China Sea (where things are going bad fast). Even calling Tibet part of China is interesting. Ukraine is Russian now by that logic.
On the ground, for the people, the real justification are different. It used to be the liberation of common Tibetan people from a regressive serfdom and a brutal theocracy, not unlike the communists' justification for themselves elsewhere in China. Similarly, after Deng, the justification is more about improving people's economic status and living conditions.
I hope this is supposed to be a joke. There is the small difference that citizens of Nevada, California and Colorado elect their representatives in the Congress and other institutions where the decisions on how to share the resources are made.
While Israel just takes what it wants by force, and the West Bank Palestinians have no say in it.