I think what you want is either plex or Jellyfin which will give you a nice UI to browse your already downloaded files.
Now how do you browse new releases and figure out what you want to download? I just setup https://overseerr.dev/ to go along with sonarr, radarr, prowlarr, nzbget, transmission... it's a lot of different services but they all work well together. Now to look for new movies, I or my family goes to Overseerr to request downloads, then plex to watch.
Same with firefox