A scene is an object. The one the scene system runs is the one that is drawn; spawned inside another scene with useSpawn, it is one more object in that scene, like a prefab. The first two games run Room (written in code) and Sign (a scene document, through sceneFromDoc) as scenes of their own. The third runs Town, which spawns two rooms and a sign as parts of itself.
One rule to know: a camera belongs to the scene that is running, and a second one replaces the first. A scene meant to be reused should not declare a camera, or spawning it would take over the host's view.