For adding new amenities - in addition to those existing like school, entertainment, etc. - do I have to simply add sub-layers under the layer "Mobility Amenities"?
OR this is not the proper way and I have to do it from elsewhere?
You have to edit the amenities profile for the project. The process has three steps: First, export the existing amenities profile using the project settings tab. Second, edit it using any text editor, by adding, removing, or modifying amenity categories as needed. Third, re-import it using the project settings tab to the project.