I have macOS Catalina 10.15.7 and want macOS Big Sur

A mid 2012 through early 2013 MacBook Pro can be upgraded to Catalina 10.15.7 but no later. See: https://support.apple.com/en-us/HT211683

A late 2013 and mid 2014 can be upgraded to the latest  version of Big Sur macOS 11 but no later. See: https://support.apple.com/en-us/HT211683

An early 2015 or later can be upgraded to the current version of Monterey macOS 12. See: https://support.apple.com/en-us/HT212735

To get the highest version of iMovie 10 that Catalina will run, use the procedure described below. For it to work, you must have, or at some point in time, did have, and earlier version of iMovie 10 on your Mac.

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