If you cannot find a safe DMG, consider these alternatives:
Disclaimer: I cannot provide direct download links, but I can guide you to legitimate sources. imovie 10.2.5 dmg
The real selling point of iMovie 10.2.5 is its integration: If you cannot find a safe DMG, consider
From a Mojave/Catalina Mac that has iMovie 10.2.5, use Migration Assistant (in /Applications/Utilities/) to transfer just the Applications folder. Sign in with your Apple ID when prompted
Cause: iMovie 10.2.5 requires certain 32-bit libraries that were deprecated in early Catalina betas, but the final Catalina 10.15.7 retains partial support.
Solution: Update macOS to the latest 10.15.7 supplemental update. If that fails, downgrade to Mojave or use iMovie 10.3 (which runs on Catalina but requires newer Metal features).