Today I finally managed to get to see what this was all about.
The way the electricity layout is planned, we have a main board next to the door (the alcove that was built earlier on) from which electricity and communication cables will go up above the decorative ceiling, run along wireframe shelves, that will be hanged from these constructions
Then the cables have to reach the main service duct that will run just below window height (both below the windows and along the side walls). To reach that duct, the cables have to be routed through the walls, and the workers excavated wide passageways and prepared tubes, one per cable.
I am mystified why they cannot be routed through a normal duct (close to the windows), but I am not an electricity engineer.
The workers has started to seal off the tubes in cement and make them part of the wall.
Oh, and the new windows have been installed, except the ones on both ends that have to be build to fit. And we got a new door frame for the entry doorway (metal instead of wood, yay!).