The Uighurs are a Muslim Turkic minority group that lives on the edges of China (in Xinjiang, closer to the rest of Central Asia than to historical China), and have for at least a millennium. The modern-day country of China consists of the historical Han heartland plus Manchuria, Inner Mongolia, Xinjiang, Tibet, and Macau. These areas are not historically Chinese, being culturally distinct with their own unique ethnic groups, religions, and traditions. China rules territories like Macau (historically Portuguese) and Hong Kong with a large degree of autonomy, but comes down quite harder on other areas, the main ones being Tibet and Xinjiang, where the Chinese government tries to erase local religion and anything else that it sees as a possible source of conflicted loyalty. So they could leave, but it'd be the equivalent of kicking all of the French people out of France.