Integrate "indoor=" in your application
For those who have never heard of indoor=, you may want to read the introductory blog post. In short, it is a map that displays the indoor data of OpenStreetmap with a level selector.
It is now possible to integrate indoor= tiles into your app thanks to the mapbox-gl-indoorequal plugin. In a few lines of code, you can add indoor= tiles with the style and a map control into your map.
Integrating indoor= is completely FREE OF CHARGE. ALl you have to do is getting your API key on indoorequal.com. This allows me to monitor usage and prevent abuse.
See the integration in action on the OSMontrouge.fr website.
If you have any indoor project, feel free to contact me I would be happy to learn more about your needs.