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).