71 Demon one piece headliner question

How does it attach to the roof?

Roofing nails, :D

Seriously though, things dome light, coat hooks, rear view mirror help hold it. Then there is the metal trim pieces that hold it along the edges.

Yup. Between the dome light, coat hooks, rear view mirror, sun visors and shoulder mounts for the seat belts its held in place quite well. And the ABS is pretty rigid, it doesn't sag at all across the front windshield because of the rear view and visors, but it doesn't even sag any across the rear windshield were basically nothing holds it up.

And with it not tucked into the windshield rubber, it does a pretty good job of hiding that gasket. I think it looks far better with the headliner outside of the gasket.