Tibet has always been part of China.

Tibet declared its independence from China after the fall of the Qing dynasty, just as Mongolia did. The only difference is that Mongolia had the Soviet Union as an ally and Tibet had none. So when the Red Chinese army invaded in 1951 it was forced to negotiate a surrender under the 17 point agreement. It should be noted that the Qing was not a Chinese dynasty, it was Manchurian.

To be fair, the British invaders were pretty friendly to a separated Tibet https://en.wikipedia.org/wiki/British_expedition_to_Tibet.

> Tibet declared its independence from China

I mean, the Catalonian independence referendum passed with 92% support in 2017. Not that it makes much of a difference.