Always verify any downloaded installer’s integrity and only use official Apple sources when possible.
If your OS limits you to the 10.1.x architecture, the App Store will offer iMovie 10.1.12 as the maximum available update. Click . Method 3: Apple Support Downloads (Archive) imovie 10.1.12 download
: If you have never downloaded iMovie before with your Apple ID, it may not appear in your "Purchased" list. In this case, you may need to briefly sign into your Apple ID on a newer Mac that can run the current iMovie, "buy" it for free, and then return to your older Mac to see it in your history. "buy" it for free