Plex is the main interface for users. It indexes media in /srv/ext/media and lets everybody watch it whetever.
The Media Sucker automatically sucks things off of CDs and DVDs. You still have to rename the files and move them into the right place, though.
Sonarr/Radarr/Lidarr/Readarr let you tell it what you'd like to have, and watches various feeds, waiting for it to appear. Then it tells a downloader to download it. Once a file is downloaded, it moves it into a directory where Plex can see it, making sure it's named properly.
Prowlarr connects to various download indexers, to find whatever it is you're looking for. NZBGeek is a paid feed provider that I use: it monitors Usenet for new posts, and provides them as feeds. I pay Frugal Usenet $5 a month to run a Usenet server so I don't have to.
Transmission and NZBGet are downloaders. Once you've found something in an index, you can ask these to download it.