If I had to guess, I'd say most of the sculpting is modeling chocolate. Marzipan is definitely a possibility, but is best suited to accents, in my opinion. More elaborate decoration is more easily sculpted with modeling chocolate, and larger decoration is often done with crisped rice and covered.

Here's something you can use as a cake topper or on its own (seeing as before we know it, it will be Halloween and Dia de los Muertos)
How to make Sugar Skulls

oh drat, now this is making me hungry