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Any video editor apps
(lemmy.world)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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Not entirely FOSS, but the Google Play variant of Simple Gallery has a built-in image editor with some basic video tools. It's using a proprietary library so this version isn't on F-Droid.