Jinping is salivating over Siberia’s resources - oil, timber, minerals, etc.
China is Russia’s loanshark, not its friend. Putin is too desperate and stressed right now to realize it.
About one-third of China is desert. China's core population also keeps an envious eye on Russia's massively wasted water resources.
Key strategies to win a game of Risk are "ally with other players for support against a common threat" and "know when to abandon your allies."