If you subscribed to Paramount+ through Apple rather than directly through the Paramount+ app or website, then you can only access your Paramount+ subscription via the Apple TV app. It is not at all intuitive either. As far as I’m aware, you have to open the Apple TV app, be sure you select “Watch Now” (Triangle Play Icon), then scroll down until you see “My Channels” where you should be able to find and select Paramount+ as an option.
You should also be able to select “Search” (magnifying glass icon) and search for a specific Paramount+ show.
I say should because some shows/episodes seem to not be working at the moment (Star Trek: The Next Generation, Star Trek: Voyager).
If you’re unsure whether you have an active Paramount+ subscription through Apple you can check by going to: Settings>(Your Apple ID Name)>Subscriptions.
I hope this helps!
-JC