To get YouTube videos offline on your phone/tablet for flights:
1. Create a playlist called “download” and add the videos you want.
2. Use yt-dlp (playlist URL) or this tool to pull that playlist to a folder.
3. Run the downloader on your Plex box on a schedule (cron/launchd).
4. Add that folder to Plex as a library.
5. Enable downloads for that library in Plex, then sync to your device.