The structure of the WTC is extremely common all over the world. None of the concrete is "hanging" and the concrete in the WTC is generally found only as floor infill and around the stair/elevator shafts. The primary structure consists of large steel columns that support large beams that span between them. These beams carry smaller beams (called steel joists) that are closely spaced (probably around 24-30 inches on center). A steel deck is placed over all the joists (basically the entire floor) and concrete "fill" material is poured on top of the deck. This results in a relatively light floor system that is structurally sound with little vibration and a great acoustic barrier.
No fireproofing works when it is subjected to a fire boosted by high octane jet fuel. With no fireproofing in place, the steel is weakened by the temperatures plus physical damage caused by the impact. As the steel loses capacity, it sags before collapsing. This collapse overloads the floor below, resulting in a progressive failure from the top down.
Well I use the word 'hanging' loosely, the point being that it's the steel frame that primarily holds the thing up, and in effect the concrete hangs on the frame, in contrast to the "large panel system", where the concrete of each level is supported by that of the levels below. Not sure what other word you would use to describe that difference in design?
The second method seems to be much more common in Europe, compared to the steel-frame approach being the dominant approach in the US, as I understand it - I wasn't suggesting the US was some sort of freaky outlier in doing it that way, but it's not how most of the post-war residential large blocks here were constructed. Just struck me as interesting that the two approaches both have a potential mode of failure, but it's different for each.
Really came to mind when the Grenfell fire happened and I saw a few conspiracy-theorists using the fact that that tower didn't fall down to argue there was something suspicious about the WTC case. When in fact the two buildings were constructed entirely differently. Also it explained to me why none of the blocks I looked at when home-hunting had gas supplies. With that sort of construction method a gas explosion could be catastrophic (in a way that presumably would not be the case for a steel-framed building?)
In addition I dimly remember some documentary about the WTC which implied there was some evidence that the fireproofing for the girders in the WTC was somehow skimped on during construction or missing in places and that this might have contributed to the disaster. Of course, a plane crashing into it, is a pretty extreme event, so it would be surprising if it hadn't caused bad effects like that.
In any case it's not clear to me what specific type of construction the block in the current disaster used.