They have a wiki which explains a lot: https://wiki.servarr.com/ as well as other arr software.
Edit: sonarr monitors your tv shows to make sure it has all episodes and grabs new ones. Radarr monitors your movie collection. It also connects directly to your torrent provider / seedbox. Can setup quality profiles and the like to customize which files get grabbed and what not. They also do bulk renaming of files to make sure plex sees them properly. Plus more.
Seedbox is optional. It can connect to different torrent software to load the torrents in. It also can just do “black hole” which dumps all the torrents into a folder that you can setup your torrent program to watch.