If you already have an older version installed, sometimes dragging the current iMovie app to the Trash (but not emptying it) and then visiting the App Store "Purchases" page can force the store to offer the correct legacy update. What to Do If You’ve Never Downloaded iMovie
Limitations
If you cannot install the latest version from the main App Store page due to OS compatibility, use this "Purchased" method to get the last compatible version for your Mac: on your Mac. Imovie 10.3.3 Download
Websites like MacUpdate , OldVersion.com , or Internet Archive may host iMovie 10.3.3.dmg files. If you already have an older version installed,
: Click the download icon. If your system is older, a prompt should appear asking if you want to "download the latest compatible version" . Confirm this to get the version that fits your OS (often 10.3.3 for Big Sur/Monterey users). : Click the download icon
: The download and installation process will begin automatically. Once it's finished, you can find iMovie in your Applications folder or use Spotlight to search for it.