Prosthetic masks are pre-sculpted appliances, made from foam latex or silicone, applied to an actor's face to physically transform their appearance in film, TV, or live performance.
The creation process involves life casting the actor's face, sculpting the character design in clay, mold-making, casting the prosthetic in the chosen material, painting it with specialized pigments and finally adhering and blending it seamlessly onto the skin.
Check out the blog written by Cinema Makeup School for a complete breakdown.