These are just the Styrene Masters. Molds were made of these, then resin casts made from the molds for the final product. Regular styrene is quite sculptable, but it helps to have a large variety of different size and shapes of stock and sheet.
one more thing, Plastic Weld glue is essential if you want to sculpt plastic. Regular glues and superglues can break free, plastic weld actually melts the plastic together, giving you more freedom to cut with pressure, and it eliminates th gap between pieces.