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.