So, unless I'm missing something it looks like you can only PIP video content..for example if you AirPlay a video from your iPhone or iPad, you can PIP, but if you just screen mirror from an iOS device you cannot PIP. Likewise, you can only PIP a video playing in an Apple TV app (that has implemented the tvOS 14 player API), not any random Apple TV app. This is disappointing, it would have been great to be able to pull up a radar app in PIP during a severe weather situation, or a statistics website in PIP sent over screen mirroring from your iPad during a football game or auto race on TV. Hopefully Apple will build on this and allow basically anything to be PIP'd, or at the very least, Screen Mirroring to be PIP'd
The content companies and the cable companies both have to play ball, and most do not like to give Apple *any* control. This is mostly out of Apple's hands
It has to use the latest video player API for tvOS 14. If they use a custom player or haven't updated for tvOS 14, it won't have the proper controls. It's not going to be premium, and doesn't require anything extra special from Apple to the devs, it just has to be a proper tvOS 14 app