Sorry if I'm too retard to realize this. But what object is she using when writing the equations?

The most coherent answer I think is she is using a big glass in front of the camera, but she would need a lavalier microphone or something like that to record a good quality audio if the glass is between the camera and her. It looks like she is writing in the camera lens instead. So, what is she using?

Her name is Zara Dar. Pic is from this video:
https://www.youtube.com/watch?v=0xO5dBja118