Protecting the North China Plain is the highest priority in China’s policies both domestic and foreign. The North China Plain is where most of the population lives (the Hu Line) and is the ancestral home for the Han ethnic group. There is also a thousand years of history that includes failing to protect the heartland (e.g. Mongol invasion).
One can explain many seemingly disparate decisions by China when looking at it through this lens. Adding borders and buffers to prevent land invasions—building the Great Wall, invading Vietnam, and occupying Tibet. Preventing invasions from the sea and protecting supply lines—cracking down on Taiwan (and likely annexing it entirely) and the Belt and Road initiative (which leads to persecution of Uighurs).
Read What China Wants.