Not who you responded to, but I can't follow a pre made campaign as a DM. My current campaign started as a pre made, but prepping for it felt too much like homework so we hopped the rails and I've been my own story.

To answer your question from how I operate, I have a rough idea of what the conclusion of the campaign looks like and a rough odea of how the party might get there. I only really know the specifics of what the party is currently doing or what they might encounter in the next months. At this point in the campaign the the players know who the big baddie is, what they're up to, and know of means to stop them. It's up to them to figure out a plan to stop the big bad. The party has taken some of my hooks to solve their problems and have gone completely off the rails at some points to solve their problems. There are some points in the campaign that are pretty linear (like right now, they need a macguffin from a particular place) and some where they're given set pieces, a location, and a good luck slap of the ass.

As it pertains to the meme up here for me personally, sometimes I'll build an encounter/boss/set piece and the party will completely fucking skip it. That's okay though, now I have a partial idea for later. I hope that gives you some insight into the other side. I'm also interested in other DMs that make their own, what methods they use.