On crows nest theres some consoles with maps that have dots over major cities, LA, washington, etc.
SO I'm guessing they obliterated most major population areas.
See the thing is...Halo has a great story, but it was poorly told.
On the humans side, they never pull the camera back and show up the grand scope of things. They never show us just how significant what were doing is on a global scale. They never show you how the covenant had been attacking earth for almost a month by the time MC got back, its population had been cut to nothing, and that you opening the hole for the unsc ships to get through was the only thing that relieved the unsc of that.