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.