The most interesting thing I learned is that there are large deposits of natural gas in the Black Sea around Crimea. They're large enough that if Ukraine was able to exploit them they'd be in direct competition with Russia and I think that explains the Crimea situation and why Russia has their best troops on that front: Putin wants to separate Ukraine entirely from the Black Sea.
My armchair general take on it: Russia wants everything east of the Dnieper and south from Transnistria to the Dnieper.
Learned that in this video by RealLifeLore: