Does mirroring use cellular data?

Ok, when mirroring from an iOS to an Apple TV, AirPlay Mirroring is in effect.

Here's where the differences between AirPlay and AirPlay Mirroring come into play for streaming:

• If the media source is located on the Internet, your iPhone would open a direct connection to the Internet source. The device (in this case, the Apple TV) would be "authorized" to access that source and no actual streaming would occur between the iPhone and the Apple TV. You can think of your iPhone as being the "control head" for the Apple TV. If your home network is gaining Internet access via a cellular hotspot, then you would see a significant data usage occur during the video playback as it's coming from the Internet.
• On the other hand and in this case, when the media source is on your iPhone, your iPhone uses AirPlay Mirroring to establish a H.264 video stream to the Apple TV. This stream is created using the phone's graphics chip and the stream uses a TCP connection over Wi-Fi to the Apple TV.

Here's some links from my notes:

• How to AirPlay video and mirror your device's screen to Apple TV - Apple Support
• Apple AirPlay & AirPlay Mirroring Explained - Lifewire
• How Apple AirPlay & AirPlay Mirroring works - FlatpanelsHD

It shouldn't as the content would already be on the phone and using AirPlay to mirror the phone to the Apple TV should only require a wireless connection via Wi-Fi.