The Indoor Mapping Data Format is Apple’s way of modeling how to map indoor spaces. An IMDF archive is one manifest.json file with many .geojson files which are tightly related to the id property in some of the GeoJSON.
Apple has created an IMDF Sandbox to validate and/or report any issues you may have with your IMDF archive.

Here is a summary of links that we used to learn development of our IMDF solution.

• The Indoor Mapping Data Format specification from Apple
• The WWDC 2019 video titled Introducing the Indoor Maps Program
  • Video 245 from WWDC 2019 describes the IMDF Sandbox

  • Because building and understanding IMDF can be a bit more complicated, there is the IMDF Sandbox, a tool for visualization, archive inspection, editing and experimentation with an IMDF archive.

• The WWDC 2019 video titled Adding Indoor Maps to your App and Website
  • Video 241 from WWDC 2019 gives sample MapKit & MapKit JS projects
  • Displaying an Indoor Map
  • Displaying Indoor Maps with MapKit JS