Since no one else has chimed in, I'm going to venture that the hearth/bench that runs across the wall is a monolithic, poured in place, cantilevered concrete slab. We've done something similar on one of our projects (below).

The fireplace facing itself appears to be plaster run all the way to the gas fireplace opening. Drywall usually won't work in this situation, because heat from the fireplace will burn the paper right off the face of the drywall, which is not good. : )