There's also Handbrake if you want a GUI.FFmpeg. It's been around for over 20 years, converts anything to anything, and is free and open-source. I use it on the regular, both on Mac and Linux. FFmpeg is like the swiss army knife of video format conversion. Tons of internet tutorials available on how to use.
I mean you can always use something like AltStore.Thumbs up to open-source software! Wouldn’t it be cool if we could download open source software on iOS?