They’re not services per se, but are a stack of software that you can kinda “clump” together for the lack of a better word.

For instance I started using qbittorrent client recently instead of Transmission that I was using before as my torrent client because I can’t find a way for Transmission to not download malware, and you can also do the very same with the indexer clients such as Jackett or Prowlarr which then talk to other software such as Radarr/Sonarr via API keys and is usually where most get caught out.

Read the docs, get thoroughly confused, and probably do it in docker too as everyone says if your hardware is sufficient.