They do it via birth control and family separation so it happening slowly but it is still decreasing the population of non-Han ethnic groups in comparison to the Han ethnic group.
When this was done elsewhere it was declared as the worst form of racism/ethnocide/genocide, but in China it is accepted standard practice.
Imagine in Donald Trump wanted non-whites to be forced on birth control in the US or if Israel wanted its Arab population on birth control -- there would be uproars and rightly so. But somehow China is getting away with this behaviour towards its ethnic minorities.