It appears to be a slab mantel finished to match the rest of the wall, but there are a couple of tricks to this. You can create a shelf from a solid slab of wood, but it is likely to check (crack) as it ages, so a slab is not ideal for a nice, stained finish. The millwork/cabinet contractor can create the mantel by building a box with a hollowed out back, which can slip over a cleat attached to the wall framing. It's smart to mount the mantel before installing the stone veneer, so remember to make the mantel a little bit deeper, allowing for some to recess into the stone.
The first photo is a slab, the second photo is a box.

This was a box construction.