When was it ever claimed in Trek that WWIII was "full-out" or destroyed "everything?" Sure, "Encounter at Farpoint" suggested that much of the world was lawless and chaotic for a while afterward, but First Contact
makes it clear that enough infrastructure survived to develop warp drive within a decade after the war's end, so clearly it wasn't completely pervasive. To all indications, we're talking something bigger than WWII but considerably smaller than the wholesale nuclear apocalypse of Roddenberry's Genesis II/Planet Earth
As far as building cities goes, Shanghai and Dubai have turned flat open fields into towering metropolises in just two decades. There are comparison pictures here
, though with some annoying ads over them.