If you own an Apple Silicon Mac (M1/M2/M3) , do NOT use 10.3.3. You are crippling your performance. You need the native version. If you own a 2015-2019 Intel Mac , 10.3.3 is arguably better optimized.
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If you have previously "purchased" iMovie (it is free) but are on an older OS, go to your Account > Purchased tab. Click the download icon; macOS will often offer to download the "last compatible version" for your specific system. If you own an Apple Silicon Mac (M1/M2/M3) , do NOT use 10
Getting Started with iMovie 10.3.3: Your DMG Download and Installation Guide imovie 1033 dmg new