@Mor - I imagine they all have their own systems of doing things. I know, for example, that David Eddings eventually started giving his wife co-author credit on the covers of his books because she contributed so much to them, but I don't think she ever actually wrote them so much as doing things like fact checking and coming up with names and so forth.
I've heard of other collaborations where big name authors will get someone who hasn't had as much exposure and give them the ideas they want in a novel then leave it to them to write it and they edit it after the fact to put it into a particular voice.
I'm sure there are others where they each take it in turns to write a chapter.
I guess everyone has a different idea of how to write a novel and what the hard part actually is. I'd love it if I had someone to write my ideas into a narrative for me because that initial part is always what I find to be the most difficult. Other people love that part and would prefer to skip the editing.