'Raw material' is collected from butchery factorites, abotoirs, farm yards (dead animals) etc and its all boiled up and cooked, bones are chopped up to make bonemeal (used in animal feed and as fuel). Fats are used for things like tallow. Because of issues such as BSE things like dead cattle are classed as cat 1 and the products produced can only end up in things that will be burnt for fuel or similar. Cat 3 (not for human consumption) tends to be butchers off cuts, etc and the proudcts produced can end up in pet/animal feeds etc, as well as, we know now... currency!
The smell of it all, if you are not used to it can be almost vommit inducing, however those who work they quickly become immune to it!.... if you ever see a rendering plant near where you plan to move you might want to visit the area when the wind is blowing the wrong way... (episode of friends springs to mind..!)