All 3D objects, POIs and tooltips are created in Online Maps. In most cases, it is more convenient to use 3D markers.

Creating overlays is highly dependent on what you have.
For example, you have a large image of pollution maps. The best way is to split a large image into tiles (for example, using MapTiler), import it into Unity and use it as overlay.
http://infinity-code.com/atlas/online-m … ample.html

Apologies for the simplicity of these questions but I am trying to understand the ecosystem of the technologies I want to use.....

I'm in the process of starting to build a 3D Augmented Reality tabletop city map with Unity. I want to be able to layer onto the map custom assets such as trees, a heat map of air pollution, tooltips on certain locations..... Are these elements made inside Online Maps or are they made in say Mapbox or Googlemaps and then that specific map is imported into Online Maps. Where would location specific POIs with clickable tooltips be created... in Online Maps or Mapbox/googlemaps etc.?

Basically, how do Online Maps and the actual mapping library fit together?

Much thanks,

Sam