How does VR headset work with Mate 20 Pro?

[JZ VR]

Bought a VR headset for $20 to $30 on Aliexpress when I put the phone in the buttons on the VR headset are able to control the phone even though bluetooth is turned off on the phone and the VR headset doesn't seem to have any power source (no batteries etc.), does anyone know how this works? Thanks.