Russia preparing for major war and large-scale mobilization

Not quite -- to keep domestic discontent low, they've had conscription. Legally, conscripts aren't allowed to be deployed outside Russia except in case of war. Mobilization means that they can forcibly draft people and seize things.

Important notes:

1. There is no war, just a special military operation
2. It's possible that the declared annexation was to be able to send conscripts into southern and eastern Ukraine since they'd be "Russian" as far as Russian law is concerned
3. Conscription could previously been dodged by paying a small fine, which many did. The government has noticed and has been steadily increasing the penalty.
4. No chance like a war to nationalize your neighbor's house