This is Alyx's photo mode.
I have no idea how to use it. The panel in the right hand always shows the same image but moves around with your hand.
The smaller panel can be moved by holding the left trigger and moving it.
This video shows how I got it working: • Guide to adding Photo Mode to Half-Life Alyx
So I think I've worked out how to use it. Move the smaller panel around to where you want to take a picture then press F12. Skyboxes are messed up and the camera frame leaves a shadow behind. Images here: https://m.imgur.com/gallery/SpzFto7