Thanks, but I was interested more in the cross-app communication aspect rather than the data formats ( https://codeberg.org/comaps/comaps/wiki/Content-Provider ).

I once looked into building an app which piggybacked on CoMaps/OSMAnd's data (saving the user from having two copies of their maps, and me from having to duplicate the UI to manage them), but found that in modern Android it was basically impossible for even the user to access data/files that "belong" to an app, without rooting the device.