I think China believes, probably correctly, that all they have to do is wait out Russia’s shrinking population for a while, and then when the time is right just start pushing here and there on the border, as they often do with India. They’ll just use their vast numbers to probe and prod at first, and then eventually invade a weakened Russia. At that point, Russia could be so weak that they just acquiesce to a gradual, non-violent colonization of some of their territory, rather than risk war with a huge adversarial army.
That is happening now and you are correct. China has been passively occupying parts of Russia bordering them with Han Chinese. Russia knows this.
No idea how they will deal with it.
An enlightened observation.
The other one is that china has no friends, maybe africans but they aren’t much help in a battle.
If the allies are defeated by china the chicoms will go after russia next or maybe just the far east of russia but go after them just the same. The need more useful land. They have about tapped out the far west of china and mongolia and the tibetian plateau.
The chinese want full bellies and can be goaded into war to get that. The fact is useful to their belligerent leaders.