The secret of the arch and dome is that the shape holds itself up with top keystones. These are amazing works in stone beautifully photographed.
The keystone part I grasp. It's the stucco designs , like these big framed floral shapes that have me puzzled. Perhaps our word, "stuck" comes from stucco....
Perhaps the whole panel was cast as one piece? Don't know. Stucco usually is attached to a wall using a backing of chicken wire or board battens. Plaster medallions are stuck to a ceiling using wet plaster, but then they are not especially heavy either. Good that the building skills still exist.
Would love to see a demonstration of the technique. Will look for such an opportunity and report back. Thanks for your continued, interesting, input.