Russia has the world's fourth-largest reserves of rare earth metals, which they're struggling to exploit due to lack of technology. China is the number one global supplier, and has all of the tech. I think they would very much like control of all their mines.

Actually taking the territory? Well, then you'd just have to look after tens of millions of poverty-stricken, barely-literate russians in the east of the country. Seeing russia reduced to economic ruin by the war in Ukraine, and then taking control of all their industry and being able to operate russia as a puppet state? Well, that sounds more like a long-term plan.

https://www.spglobal.com/marketintelligence/en/news-insights/latest-news-headlines/russia-struggling-to-capitalize-on-rare-earth-reserves-52525919