They are two different programs. Prowlarr is an index manager and overserr/jellyserr are requesters.

Indexer pulls a listing of available files from as many indexing services as you add. 1337x, NZBgeek, etc.

Overserr is like a front end for your entire stack. You just tell it what you want media you want and it sends the requests to the stack to figure out.

Overserr talks to -> radar/sonarr talks to -> prowlarr talks to -> sabnzbd/qbit.