Not a designer, but pretty sure that is not wood but tile made to look like wood. Notice how it also extends into the shower area.

Perhaps they used a cleat system on the back of mirror & tile wall or maybe it is hanging off bolts. If it is glued, there clearly seems to be something between the mirror & tile.

If you have a wood wall & want to hang a mirror out from it, & if it isn't framed and has no backing, try gluing something to the back of the mirror (like wood) with a cleat already attached to it, then attach the other part of the cleat system to the wall, then slide it on.