Searching for on random file-sharing blogs exposes your Mac to severe security vulnerabilities, including malware, adware, and data theft. Because iMovie is a completely free tool provided by Apple, the safest, smartest route is to utilize the Mac App Store's "Purchased" tab to pull the last compatible version for your specific system. Avoid third-party installers to keep your creative projects—and your private data—completely secure.
If you are running an older OS like Big Sur or Monterey and cannot update to the latest iMovie version, a standalone DMG is the only way to install the app.
Before installing an older version over a newer one, back up your .imovielibrary files to avoid database corruption.
If your Mac cannot support the latest iMovie version, a prompt will appear asking: "Do you want to download the last compatible version?"