From my understanding, they use the fish's skin and fins and other external bony parts such as the jaw. I think there is a wood form in the middle and the rest is saw dust or something?
There may be other ways too. Someone told me that some taxidermist can make a re-creation using fiberglass and that uses none of the fish. If anyone knows, I would be interested I would love to have a realistic model of my old Dat. I guess I can google it.
they use the fish's skin and "stuff" the inside with foam.. then they apply a sealant/clear coat on the top to help preserve it better.. at a reputable taxidermist ive seen fish that are 30+ years old and still look just like the real deal.. i have 3 myself but are not my pet fish..