The GitHub explains it great but in summary you mount your Real Debrid (or other Debrid service) account to your machine via rclone, so anything you’ve downloaded on Debrid is available on plex.
Then there’s a script that watches the plex watchlist for new requests and then scrapes torrents to get it. (similar to stremio): If it’s cached (already available on real Debrid) then you can watch it instantly. Which is majority popular content. More niche stuff it may have to request Debrid to download and try again later
Then refreshes plex to scan the library for the download.
Even works outside of plex with things like overseerr, jellyseerr, trakt to monitor requests. Also can use Trakt to manage your library. Even Support for other scrapers like jackett and torrentio and you can set it to refresh your jellyfin library if you aren’t using plex.