Open the app
Tap New Map
Place all the objects your heart desires
Pay attention to the Save Button, it will only turn blue if the map is good enough to save, Scan your environment until the button turns blue.
Click Load Map
Point towards the area where you built your city
Mapping (more info here)
Saving (more info here)
Relocalization (more info here)
Minimum map quality threshold (more info here)
ARkit Hittesting (more info here)
Plus a new concept:
Meta Data Usage by saving the positions of multiple objects in the scene.