/jp/ - Otaku Culture

gonna use thread to ask because I don't a point making a new thread for this:
What exactly is "doujin culture"?
People in Japan can make derivative works of any IP and sell them at conventions? Be it music, comics, games, etc.?
Say, a guy makes a music album using different instruments compared to the original, can he sell that at conventions freely? Or the IP's owner has all the rights to decide what happens to that music album?
What about exceptions like Nintendo shutting down fan projects? Isn't that against the whole "doujin culture" that people like?