The *arr services are fantastic for helping you organize your content as you download them. They won’t find anything that you wouldn’t be able to find yourself, though. You feed it your indexers via Jackett or Prowlarr for torrents, or NewzNab (or equivalent) for Usenet. What shows up on my Sonarr searches is likely very different than somebody else’s since we likely have different indexers.
If you are looking for content that is hard to find I would recommend getting into Usenet or joining a private tracker.
If you want to get into building a large library and hosting your it on a Plex/Jellyfin/Emby server, I would recommend getting into Sonarr and Radarr, and then learning about the other *arrs.