China will not annex Russia or anything like that. But their government has everything to gain from extracting concessions from Russia. Russias influence is already waning in Central Asia, and China is eager to fill that role.

Russia is on its way to becoming hamstrung by China. Who else can Russia turn to?

China does not want Russia to implode and fracture, because that is an unstable situation for them. But if Russia was to fracture, you can be sure that China will want to control the pieces bordering China. They don't want chaos on their borders, or the power vacuum there somehow filled by western powers.