Have a look through the tools section on the Megathread in the pinned post. For this specific use case you're probably going to be wanting to look at tools like Sonarr (for TV shows), Radarr (for movies), some form of torrent client that those tools support (Transmission for example), and depending on what your tracker supports, possibly something like Jackett to provide a bridge between your tracker and your downloader tool.

The benefit of this kind of setup is it's very easy to add Usenet into the mix if you choose to.

There's some extra steps needed if you run it directly on the Mac but you can also do something like run Docker on the Mac and run those tools within Docker instead.

I'm pretty sure it's possible to integrate something like Overseerr (which is a web frontend for handing requests for new content) into the Plex watch list meaning you could add a show to your watch list in Plex, Overseerr would pick that up, send it to Sonarr or Radarr depending on the type of content it is, which would then do a search on your tracker for the content, send the torrent to your torrent client, and then when it finishes downloading automatically import it into Plex.