Adding to what Niel posted:
I don't know about Final Cut, but if you have, or ever had, a version of iMovie 10 on your Mac you can download the highest version of iMovie that your present operating system can run with the procedure below. Otherwise you will need to upgrade your operating system to Catalina or Big Sur to get current iMovie.
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