It uses wifi directly with the Apple TV. Which needs, after doing some research, to be 3rd gen rev A or later ATV. Basically the latest two.
I am using an iPhone 7+ and ATV 4 and able to mirror the entire iPhone home screen as well as various apps including YouTube.
It appears only wifi needs to enabled on the iPhone. Of course on the ATV as well as that initiates a peer to peer wifi connection.
This guy states you need Bluetooth enabled on the iOS device. But mine is working without it. You can play with it though.
Since your iPad is LTE, it can actually connect to the internet to grab the YT videos or what not. If there is no LTE connection, then only content on the iPad can be viewed. YT allows to download videos before hand, but I believe that's a paid service.
