There are different ways to build regions in space.
You can build region assets in other software packages and import into Unity.
You can use Unity's mesh shapes and terrain tools, and you can grab third party assets from the Unity Asset Store.
All of the pages linked here all refer to the Scene Export Settings/Scene Save Settings component:
Assemble your scene
You can upload any scene you create in Unity as a region in Space.
Once your scene is assembled you have two housekeeping tasks;
Remove Main Camera
When you create a new scene in Unity it will by default have a light and a camera.
You need to delete the camera (by selecting it in the Hierarchy window and clicking delete).
Add Landing Zone
You need to define where users arrive when they log in to your region; and what direction they are facing. To do this, once your scene is set up, from the top menu in Unity select Game Object / Create Other / Landmark.
If your Scene Heirarchy menu does not include Create other / Landmark, then;
Create empty object, then in the Inspector, add "Landmark behaviour" component.
This will place a new landmark item in your scene. In the Inspector window, set the type to Landing Zone and tick Spawn Point.
The in scene object is a yellow edged transparent plane which you can position anywhere in your scene.
The yellow arrow points in the direction your visitors will face when they arrive.
Be careful to position the landing zone slightly above the terrain; if a tall avatar's feet arrive below the terrain they may fall through.
Adding a New Scene to your Region
This does not apply if you are using Multi-Scene Mode & Multi-Scene Editing.
You can have more then one scene in your region.
After placing your landmark, go to 'Space' at the top of Unity, and choose 'Scene Settings'.