As per the video
https://youtu.be/If61baWF4GE), above and beyond the geopolitical reasons for Russia's invasion of Ukraine, the main motive is economical:
1. Russia is in effect a petrostate; it lives and dies by its exports of natural gas and oil.
2. Immense fields of underwater natural gas and oil were discovered in 2012 in the Crimea maritime exclusive economic zone.
3. If Crimea stays with Ukraine, it lets Ukraine become a new petrostate and challenge Russia (take over its European customers), which to Russia is an existential threat. Hence the annexation of Crimea in 2014: not only for Sevastopol and access to the Mediterranean, but to exploit the oil fields (and/or prevent Ukraine from exploiting them).
4. But the supply of 80% of the fresh water to Crimea comes from a canal [0] that connects it to the Dnieper river in Ukraine; after the annexation, Ukraine shut the canal down, effectively thirsting Crimea to death.
5. If Russia wants to stay in Crimea (which it believes is necessary for its own survival), it needs to reopen that canal, and therefore control at least the southern part of Ukraine and possibly its whole Mediterranean coastline.
(It's not just about the canal, but the canal is a very big part of it.)
[0]: https://en.wikipedia.org/wiki/North_Crimean_Canal