Where does the Scrum Backlog come from?

Scrum is easy to understand, isn't it? The Development Team agrees the to-be-created User-Stories of the next Sprint with the Product Owner – and that's it. Really? 

But, where indeed do these User-Stories come from? Is it accurate to claim that this is merely the maintenance of a list of stories? This topic is dealt with by Jeff Patton in his highly recommended book "User Story Mapping" (Amazon link).

To reveal one insight from the book: it doesn’t make sense that an agile project (according to the SCRUM framework) starts by writing little cards, discussing them, doing the first Sprint planning, and then just letting it run.