upgrade from 10.15.7 to 11.5.1 or higher

Depending on which model of MacBook Pro you have, you would need at least a late 2013 or later in order to run Big Sur. Here's a link that lists the Macs that can run Big Sur:

https://support.apple.com/en-us/HT211238

Since you have a MacBook Pro 2012, I am afraid that your Mac is too old to upgrade to Big Sur in order to obtain the current version of iMovie. If you have, or ever have had, an earlier version of iMovie 10 on your Mac, even if you had deleted it, you can obtain an iMovie update to the highest version of iMovie 10 that your Mac can run. If that's your situation, you can try the following procedure:

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the

trash (if you moved it there) and return it to the Applications folder.

-- Rich