If you go to the App Store and see "iMovie requires macOS 11.0 or later," you have two choices:

Finding a direct DMG download for a specific older version of iMovie like

If all else fails, explore the alternatives listed above. Many open-source editors now rival or exceed iMovie’s capabilities while supporting older operating systems.

or later. If you are on a much older OS (like High Sierra or Mojave), you might actually be looking for version 10.1.12 or earlier.

Apple does not provide official DMG (Disk Image) downloads for modern versions of iMovie like . To get this specific version, you must use the Mac App Store

On a Mac still running macOS 10.14.6 (Mojave) or 10.13.6: