I find mine all over the island except where my campsite is. I made the campsite area all beautiful and stuff, but I've never seen them go near there! The only ones that I've seen on my third level though are the 2 that live up on the mountain...
I noticed the past couple days they like to congregate wherever I'm building, which is really cute (and sometimes annoying). Anywhere I'm laying down paths and placing furniture items, they'll walk by and interact with the stuff I'm putting down. Even terraforming! I started digging a pond the other day and villagers kept plonking themselves down to enjoy the pond before I finished, lmao.
So yeah, it seems like they like being in the more developed areas as others have said, for sure.
My villagers have been found all around the island, and some seem to wander more than others. Raymond, who lives very far southwest, has been seen in the northwest, right next to my house, and the southeast, at the beach. Octavian seems to enjoy sticking around his house and the trees.
As I first started the game, prior to building any inclines, I saw Mira on a cliff that would be inaccessible without a ladder.
There seems to be only one part of my island my villagers never visit, that's the eastern side
Even sterling who lives over there never wanders near the swamp in the top right or the beach, it's gotta have something to do with paths cause those are the areas I have the least paths