Apple TV+ is a channel offering Apple-provided streaming content. That’s what you get. You don’t get content from other content providers.
The confusingly-named Apple TV app and the confusingly named Apple TV 4K box can stream both the confusingly-named Apple TV+ content from Apple, and can stream content from many other less-confusingly-named providers.
The Apple TV+ subscription, like a subscription from pretty much any other content provider, only gets you that associated content.
Cable TV providers and cellular providers can have streaming content packages that can be accessed via the Apple TV app, or the Apple TV 4K or Apple TV HD set-top boxes, using a login provided by the content provider.