Boolean has to be done before mirror. The modifiers go in order from top to bottom. So it’s mirroring the solid piece before the holes are cut by the Boolean.
***Edit*** To make myself more clear. You have the cutting object only on 1 side. you can either do the Boolean first or add a Y mirror to the cutting object. I can’t remember, but I think he leaves the back blank like what you have and cuts USB sockets in the back.
