A little of column A, a little of column B. DM's often use pre-written campaigns when they first start, and gradually work up to fully self-imagined campaigns. Because of the massive amount of player freedom a DM has to be prepared for anything. One of the DM's I know was telling me about a campaign where the players spend most of the time trying to sell large amounts of wine rathere than actually progress the story.
It’s basically “here’s the main villain. You gotta kill him or the world ends.”
Players: “ok, but first we gotta find this little kid’s goat called Jamesy. It’s much more important.”
Players: “ok, but first we gotta find this little kid’s goat called Jamesy. It’s much more important.”