They don't make it easy to find, do they?
The naming doesn’t help either, imho.
So, the rest of the stuff, like Hulu, do you need a Hulu subscription, or are you purchasing the Hulu subscription thru Apple? What if I already have a Hulu subscription, can I link it into Apple TV+?
Hulu will require a separate subscription and a separate app. You can subscribe in the app through your Apple ID account, or with Hulu directly (through their website), which would require a separate account with them.
Hulu can be linked to the Apple TV app (that is: Hulu content will show in Watch Now and Up Next in the Apple TV app).
Apple TV+ is a separate Channel in the Apple TV app. Nothing “links in to” Apple TV+. It is its own content (collection of shows) with its own subscription.