`pkg install ffmpeg`

It's a thin wrapper around apt for whatever reason. Quite a lot of software packages are distributed by/for termux!

There are number of different places to get this info. I use Termux with yt-dlp and decode to MP4 or extract audio only which requires ffmpeg and it works fine if a little slow. I decoded a 300 GB twitch stream that after downloading took about 20 mins to complete the decode (encode?) to mp4 on my Pixel6. But it works perfectly.

e.g see https://gist.github.com/cyrillkuettel/d63785cf5f4c00106ae215... for example.

Pixel6 "only" has 256Gb storage, how did you download 300Gb?

pipe yt-dlp into ffmpeg, set encoding setting such that it produces an output file small enough to fit onto storage or external storage

Can't you just download the lower-res video using yt-dlp, or isn't that a thing on twitch?

Termux has an open source repository of packages. It's similar to arch, debian, alpine, etc. https://github.com/termux/termux-packages

Termux doesn't get special privileges unless you root your device. It can't go messing with your phone data unless you granted it permission to do so (but even then it doesn't have access to everything). It's no worse than any other Android app IMHO.