This is the most reliable way to get an older version if you have downloaded iMovie before. Mac App Store Account Name (bottom-left corner). Find iMovie in your list of (cloud) icon.
safely, you should use official Apple channels, which often allow you to download the last compatible version for your macOS. 🛡️ Risk Assessment Security Threat : Third-party DMG files for iMovie are frequently unofficial imovie 1033 dmg hot download