Russia haven’t been able to fully mobilize their population for war. If Putin can convince Russians that they are in an existential war with NATO then he can introduce conscription. If he’s able to double/triple the bodies on the front line in Ukraine then there a real possibility of overwhelming the defenders.

This isn’t new, there were Chinese mercs fighting in the first couple of years of the war.

https://www.newsweek.com/chinese-mercenaries-russia-ukraine-war-1876765

There was also that video doing the rounds of a Chinese merc asking his government to pull him off the front line.

Why would either need to side with Russia? They only have 140m people, a untrustworthy and soon to be unstable government. If you’re aim is to carve up territory then you don’t give a potential long term adversary access to half a billion people.