That’s supposed to be part of a cross-section of a wall lining with two layers of plasterboard (grey) with a gap between them (say 25mm) for cables, pipes etc. I’ve just seen builders next door constructing something very much like this.
Red thing is a back box for the switch (or socket) in green.
Looking at metal back boxes, they all seem to have fixing holes on the back, and near the front edge of the sides. In my sketch, I’d like to secure the box to the wooden batten (blue). But none of the holes is in the right place.
So do boxes in this sort of construction just get screwed into the back layer of plasterboard?