Why doesn’t China ally with Ukraine and invade Siberia as Russia is severely weakened?

Because China is smart enough to reach its goals in less expensive ways.

China does not need paper territorial gathering. It needs money, cheap resources, stable supply, and, the most important – favorable international climate, which does not distract it’s trade. The war could jeopardize it all. Alternatively, they have it all now without a war. Russia lost huge part of its sovereignty and literally became economical appendix to China. They sell gas, crude oil and other raw materials very cheap while purchasing vast volume of ready-made goods, especially high-tech at enormous prices. So why China loses all of this and enter complications of the war?

Amazingly – America could be the one who stand on the Russia side in that case to prevent China from growing.

On top of this – although China is 10 times bigger by population it’s active military personnel is not as big and the war won’t be easy for them although they have big advantage in the long run. Furthermore, in case of really big strategic loss Russia can use nukes, which makes such territorial gathers useless and way too expensive for China.

At the same moment – they buy Russian elites making them pro-Chinese and implementing respective economical laws. They have big Chinese diaspora in Siberia, which continues to grow. They perform slowly expansion in the region right now. They are conquering Siberia – but in a slow and the most efficient way without all these childish war games.