Improv DM or Prep DM?

What type of Campaigns do you run??

I have a mostly improved Campaign going on now- I prepped a ton of stuff for the starting City, even got monsters and important locations in the world. My players keep driving the story in crazy directions and it's absolutely hilarious. I am having an absolute blast as a 1st time DM and just curious how everyone else builds their world and the lore within.