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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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I believe this is the only one on fdroid:
Video Transcoder (Video transcoding between common formats) https://f-droid.org/packages/protect.videoeditor/
its not been updated in 4 years, and its clunky, but it does work. If you absolutely have to go opensource, this is ypur best option.
Otherwise, simple gallery (pro version) has a video editor, but its propriety, so only on the play store version, not fdroid