If your Mac runs Catalina or Mojave, open the App Store → search iMovie → download. Apple will automatically give you 10.3.3. If that fails, use mas install 408981434 in Terminal. Do not trust third-party download sites.
: iMovie 10.3.3 is the primary version used for systems like macOS Monterey (12.x) Apple Support Community is compatible with 10.3.3? Imovie 10.3.3 Download
If you don’t specifically need 10.3.3, Apple’s latest iMovie is free and superior in every way (speed, 4K/8K support, HDR, etc.): If your Mac runs Catalina or Mojave, open