That depends on where you are in the world and the limb that you have. It’s pretty standard that the socket (the part that attaches the limb to the body) is made of plastic in the developed world as it’s easily shaped and pretty sturdy. However, the more expensive ones are made out of carbon fibre because it’s lighter. The limb itself is usual some form/combination of metal.
In low-middle income countries it’s not uncommon to use wood for the limb and socket because it’s cheap, but this is very heavy and can rot. Though metal and plastic are also used.