Jackett works as a proxy server: it translates queries from apps (Sonarr, Radarr, SickRage, CouchPotato, Mylar, DuckieTV, etc) into tracker-site-specific http queries, parses the html response, then sends results back to the requesting software.
Overseerr is a free and open source software application for managing requests for your media library. It integrates with your existing services such as Sonarr, Radarr and Plex!
ruTorrent is a front-end for the popular Bittorrent client rtorrent.