Extrapolating from bits I've read online (which means, this is probably completely wrong... but possibly not!)... Most of Russia's income is from oil and gas.

Extrapolating from bits I've read online (which means, this is probably completely wrong... but possibly not!)... Most of Russia's income is from oil and gas. And the profits from oil and gas are received by a very small few people. Those people probably do not pay a lot of taxes. Therefore, it can be assumed that the Russian military and related systems are not well funded. Russia has for decades had a brute-force approach to military - simpler, less efficient systems which are more robust and which can be field serviced. If numbers found on the internet can be believed chuckle, in 2017 Russia spent 66 billion USD-equivalent on its military. The US spent $646 billion. Now maybe the US is much less efficient, with grifter middlemen contractors, but still... 10x the expenditures. As for China, they want the energy and mineral resources from Russia. They are treading carefully to appear supportive of Russia while not alienating the rest of the world (their major trading partners).