Where does it come from and what kind of terrain model is used in the app (DSM or DEM/DTM)?

Please, consider using a newly released AW3D30 model from the Japanese space agency.

It's free (requires registration):

AFAIK, it's the most recent topographic data publicly available (the latest update comes from January 2021) and it uses a DSM format (e.g., includes vegetation and man-made structures, which is a good thing for pilots). I know first and foremost it's a VFR app, but having more information is better than not enough (when you unintentionally flies into IMC, to make sure you don't smash into anything).

I definitively plead for fit-for-purpose and only good-enough data which is small data sizes and not over-structured ground data. We don't need 1" details on the ground, which is totally irrelevant to flying, plugging data processing with no real benefit at all and harming performance. Like daytime running lights in violation of climate rescue delusions ...