Downloaded files are stored in external private storage (Android/data/com.github.libretube). Note that since Android 11, the READ_EXTERNAL_STORAGE permission does not grant third party apps access to external private storage, requiring workarounds. The (usually hidden, not to be confused with Files by Google) Android built-in Files app will always have access to this directory. In the downloaded files, audio and video are separated. Hence it is recommended to exclusively use LibreTube for playing downloaded media.

From libretube FAQ

It's not simple with LibreTube, as audio and video are stored separately, but you can use yt-dlp with Termux to download YouTube videos normally.

Here is an alternative Piped link(s):

https://piped.video/jrZtp9vuWPM

Piped is a privacy-respecting open-source alternative frontend to YouTube.

I'm open-source; check me out at GitHub.