How does the Apple TV remote control my iPhone when using Airplay? Today I discovered something cool while using Amazon Prime + Airplay to Apple TV 4... I discovered that I can use the AppleTV remote to fast forward and rewind. 


Does anyone know how this feature works?
Does an AirPlay session allow for more than just "broadcasting" to the target device, or is there some kind of back-link to the phone? 
Does the phone "see" the Apple TV remote? 
Does this mean I can use the Apple Tv video game controller in conjunction with my iPhone while AirPlay is on?
Applications can respond to the "Remote Events" from the AirPlay device. This is some kind of back-link as you described it. Developers has to respond to those events manually (unles they are using some built in classes..). For more info please check official Apple site.


  When AirPlay is in use, your media might be playing in another room from your host device. The AirPlay output device might have its own controls or respond to an Apple remote control. For the best user experience, your app should listen for and respond to remote events, such as play, pause, and fast-forward requests. Enabling remote events also allows your app to respond to the controls on headphones or earbuds that are plugged into the host device.


So your phone is "not aware" of the remote/game controller etc. It receives some events from the AirPlay.

For the allowed events please check developer site.