China and Russia have no military alliance, Xi is just taking advantage of the fantastic business opportunity that opened up for him for cheap Russian imports. If Chinese troops became militarily involved in Russian territory, it would probably be to acquire their claims over eastern Siberia, dating back to the Qing Dynasty. Otherwise he’s just be supporting a state that competes with him for the dominant regional position.

The Tsar took that land from them, back in the day though. If you look at their current claimed territories, they never forgot. Maybe if Russia agreed to become a subject state of China, then they’d help militarily, but they’re certainly not friends or allies.

Russia does have actual allies, but not many. And China isn’t one of them.