Passthrough Mask in Unity with Quest 3

I've been trying to figure out how to replace the passthrough video ceiling with a VR scene like in the First Encounters demo or the Discover app on Github. I've tried working with the layers and copying the shaders from the Discover app, but haven't made any progress. Does anyone by chance know how to mask the passthrough video to reveal the VR scene underneath? I would be super grateful for any advice. Thanks!