Again by extruding edges make the place for the supply the main fan and the places for video, sound card etc

It is easy to create all the little part at the back so I’m not going to explain it just see the pictures and try to do something like that:

We are going to create some holes for the second fan. Select the polygon where you want to make the holes for the fan and click a few times on the tessellate button. This will add some more polygons.