Could you tell me what are casts for broken bones made of?

Fiberglass: Most these days are fiberglass. Plaster of paris is still used by some. There are advantages and disadvantages to each. Fiberglass is "waterproof" (but one has to use a hairdryer to drive the water out to avoid skin irritation). Plaster tends to be less expensive and easier to mold, but has to be kept dry.

Many things. : Casts can be plaster, or fiberglass. Sometimes we use a half-cast, or splint, or even a removable brace for a hand or foot.