You can read about what each module does in the official TextFormatter documentation.
AutoVideo transforms links to video files into a standard web video player. URLs must end with .mp4, .ogg or .webm file extensions and point to a video file https://s9etextformatter.readthedocs.io/Plugins/Autovideo/Synopsis/
MediaEmbed transforms links to specific websites into their embedded version. YouTube is one of the supported websites https://s9etextformatter.readthedocs.io/Plugins/MediaEmbed/Sites/
If you only want to enable YouTube and not all of the other supported embedded sites, you might find this extension useful https://discuss.flarum.org/d/31404-selective-mediaembed
