How do I correctly add subscriptions through Apple TV?

Some streaming services have in-app subscribe options in their own app, billed through the iTunes Store. Then watch in their own app.

Some streaming services want you to subscribe directly with them (often on the web). Then watch in their own app.

Some streaming services partner with Apple through an Apple TV app Channel, billed through the iTunes Store. Then subscribe in the Apple TV app and watch in the Apple TV app.

Subscriptions through Apple will be linked to your Apple ID account and be recognized automatically on Apple devices.

Subscriptions through the content provider will have their own account credentials.