How To Remove old login screen of MacOs?
Seems like somehow you're stuck at Mojave's Preboot environment.
With FileVault 2 on, the Preboot environment (the Mojave one that you see), instead of full-fat macOS, is booted first to ask you for your FileVault 2 password in order to decrypt your data and then, proceed to normal macOS startup procedure (when you see the Big Sur startup interface).
The strange thing is that the preboot environment itself should be upgraded along with your recent Big Sur installation, but wasn't. If you're using MacBook Air 2018 or later, T2's magic may be to blame.
The solution is: manually updating your preboot environment. Here are a few suggestions:

Enter macOS Recovery, and

Install Big Sur again
OR, if that still didn't work, turn off FileVault 2 and:


In macOS Utilities, open Terminal.
Run sudo diskutil list to get Preboot's identifier (usually disk1s3).
Run sudo diskutil apfs deleteVolume IDENTIFIER.
Install Big Sur again like in method 1.



Either way, your data should be unaffected. But be sure to make a backup in case something bad happens. Make sure to turn off FileVault 2 and wait for it to get fully decrypted before proceeding to recovery method 2.