Occasionally, Apple provides standalone updaters for professional applications. While rare for iMovie specifically, it is worth checking the Apple Support "Downloads" page for "iMovie 10.2.5 update." Note that these are usually patch files that require a base version of iMovie to already be installed, rather than a full DMG installer.
Be extremely cautious of websites offering "exclusive" or "free" iMovie DMG downloads outside of Apple's official channels. imovie 1025 download exclusive dmg
When Apple updates iMovie in the Mac App Store, the platform updates the system requirements. Users stuck on older operating systems like macOS Big Sur or macOS Monterey often cannot download the newest iMovie version. This restriction drives them to search online for legacy .dmg (Disk Image) installers. The Hidden Dangers of "Exclusive DMG" Downloads When Apple updates iMovie in the Mac App
You can keep the installer on an external drive for future use. System Requirements for iMovie 10.2.5 The Hidden Dangers of "Exclusive DMG" Downloads You