Hey Lorechat! I just saw the trailer, and I beleive I have found some understanding to the many cultures of sky.
The Isle, is a lush prairie who intersects with the forest, here we see, maybe, three groups of cultures.
The first are the Cavern People, Stargazer, Voyager are seen living in a dimly lit cavern. The Cavern people in game created the Cave of Prophecies, to live in it. These people prob invented the murals.
Second is the boat people, who have tents and boats and huts lined up. They might've migrated massively trough forest and end up colonizing the Valley and Wasteland. (however due to Candlemaker having a boat and living in a a cavern maybe boat people are a derivative of more liberal form of Cavern People.
Third are the 'Travellers' which would be travellers flying around in Krill related light creatures,who would travel troughout the realms as nomads.