I have a favourite way of making plot, and it’s to write down a few sentences about what’s likely to happen if the PCs don’t intervene, add a few nodes for the most obvious things they CAN do, and then make things up from there. The good thing about this is that your adventurers will inevitably discuss what they think is happening, and sometimes they’ll have a better idea than you have, and you can just switch over to that being the plot.
It’s one that I’ve heard of being used in Call of Cthulhu, where you run a session, let them panic, and then take the worst thing that they think of and develop that as your bad guy for the next session. I’m a lot nicer than that, at least when I’m not GMing CoC. Sometimes their plotline is just clever, sometimes it fits into the world, sometimes it explains things better than you could.
It doesn’t happen all of the time. Most of the time I’m extemporising based on what I think will happen, but the best plots usually come when a player or a character makes a great guess, and I ignore what my notes say, and they turn out to be right.
I have a great set of players.