Connecting streaming apps to apple TV+ app

What is your version iPadOS?

Get the latest release version for optimal support. We can share how-to information for the current version, which might differ for previous versions. Update your iPhone, iPad, or iPod touch - Apple Support

Please differentiate between connected apps and in-app Channels.

• Connected apps share content with the Apple TV app for Watch Now recommendations, the Up Next queue list, and Search. Playback will then happen in the content provider app, which has to be installed on that device. This system works on iPad, iPhone, iPod Touch, Apple TV box, but not on Mac or third party devices. Availability of supported apps in the Apple TV app - Apple Support
• Apple TV app Channels offer select third party streaming services with Apple as content deliverer. These Channels require their own subscription, which is not exchangeable with the separate streaming provider app. This works on any device with the Apple TV app. Subscribe to Apple TV channels in the Apple TV app - Apple Support

Users may manage (on|off) connected apps through the in-app account settings in the TV app on the device: Connect video streaming apps - Set up the Apple TV app on iPad - Apple Support

Subscriptions through Amazon may need to be watched in the Prime Video app. I don’t think those can be connected to the Apple TV app. Connecting the Prime Video app only covers titles that are included with Prime, not additional purchases or subscriptions.