The concrete looks as substantial as a sub-standard chocolate Easter Bunny. It looks like there’s a frame in there, but not really integrated with the concrete, so when the concrete cracks, it’s done for.
I think I’d have a frame supporting a heavy wire mesh, and then hit that with shotcrete, followed by the artist with sander and chisels.
Of course, what you really need is a giant 3D head printer.
My guess would be fiberglass on a steel frame with a coating of concrete/plaster resin.
Clearly these were never built to last any longer than your average theme park attraction.