Newer iMovie versions require macOS 11.0 or later. iMovie 10.3.3 runs smoothly on:

While some users seek third-party .dmg files on sites like Reddit or Archive.org, be extremely cautious. Only use reputable community archives to avoid malware. Version 10.3.3 Highlights

Keep in mind that iMovie '09 is an older version of the software, and it may not be compatible with the latest macOS versions or hardware. If you're looking for a more recent version of iMovie, you can check the Mac App Store for updates.