Er they already have a corridor across the Kerch Straits with a bridge that takes Road and Rail traffic.
I’m aware of the bridge - and Russia stopping Ukrainian and other shipping going under it in contravention of the right of free passage (The crews just having been freed) - but bridges can be cut, which is why a land bridge is preferable. And of course preferable if you want to link to Moldova and limit Ukrainian access to the sea.
One of the reasons why Crimea is mainly inhabited by ethnic Russians is because of the ethnic cleansing by deportation of the native Crimean Tartars started by Stalin in 1944, continued by the policy of detartarization under Khrushchev in 1956 and continued oppression and suppression under Putin.
The seizure of Crimea, of course, having been condemned by the UN and is unrecognised except for other Russian client states such as Cuba, Syria and Venezuela.