The Apple TV app supports select third party in-app channels on supported devices, but those channels require subscribing through the Apple TV app. Those channels are not compatible with existing subscriptions through other parties or other apps. Channels is not like installing apps within apps; that is not supported by the host OS.
Subscribe to Apple TV channels in the Apple TV app - Apple Support
(The same applies for in-app channels through Amazon Prime Video app.)
To use this existing direct subscription, you will need a device that supports having the MGM+ app installed. That can be a supported smart TV or an external streaming device, such as Apple TV box, Android TV device, Fire TV device, or Roku device.
Get the channel or the app - Stream on your favorite devices - MGM+
- Else you may consider canceling and resubscribing in a different supported manner.
Apps on Samsung TV require that a third party app developer submits an app to the App Store of the device manufacturer. That would mean a different app edition for each platform (operating system).