The Twin Towers had a vertical "core" of support girders in the middle of them.
And they also had a vertical "perimeter" of steel girders which also supported the vertical load.
Between that was open space.
Horizontal girders (called floor trusses) were attached to the vertical core girders on one side, and to the vertical perimeter girders on the opposite side.
The concrete floor slabs were poured and positioned to rest on top of the floor trusses.
And all these steel girders were sprayed with a layer of fireproofing material.
When the airliners struck and entered the buildings, the fireproofing material was knocked off many of the floor trusses in the impact zone. We know that because observation of the floor trusses in the rubble pile showed us exactly that.
When steel is heated, it weakens. That's why they fireproof it so it will retain it's strength in case of a building fire.
As the floor trusses in the impact zone heated up from the fires, again with the fireproofing material now missing, they began to weaken and over time they started to sag. And I mean significantly sag.
This photo shows us exactly that...
Eventually, the sagging puts enough stress on the riveted joint where it's attached to the vertical support girders holding it up, that the floor truss breaks away from the vertical support. And that's all she wrote.
When that happens the truss falls, the additional stress causes other trusses to break away and fall, and the massive floor slab it was holding up comes down with the falling floor trusses.
All that falls onto the next slab, and the next and the next and so forth and so on.
And of course gravity then brings the whole shebang down to the ground into a pile of rubble.
There's no mystery about it and it's not complicated.