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
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.
Termux + ffmpeg
Haha best answer unironically