Shit, that's the part of prep that's fun
Ignoring established world building, which I don't count as campaign building, I tend to have an overall plan, plus a handful of probable diversions. Each path is going to have NPCs as appropriate, though I tend to keep them as just notes until maybe a couple of sessions before they're likely to be used.
As an example, I might have "lvl 10 chronomancer; drow, w/magic staff, 1 ring, and familiar" when I rough out the campaign. Once I'm sure the players are going that direction, I'll pick the specific gear, build the NPC's spell list, and have a plan for the encounter that includes combat, non combat, and usually a combo of the two. But the encounter plan is all mental, no need for written notes of that kind of thing.
If they're going to a location I've already got on paper, it's easy prep. But I do have a lot of room that isn't labeled on the map, as well as locations nobody has gone during play. For those, it's usually just writing down existing ideas, and cooking up details if there's a divination or scouting run, or whatever.
It's one of the benefits of having an established world to play in that's also huge enough that there's room to grow. I never have to build from the bottom up, it's only for one off and/or "non canon" play that I do much of that at this point. So most of the depth of those is in my head anyway, not on paper at all.
Mind you, I still have worlds I haven't used, and likely never will. It's a long standing mental hobby. But I really enjoy both depth and breadth for my setting.