In many cases OSM-based solutions like Organic Maps or Mapy.cz are good enough (though for car navigation Google Maps is still clearly better).
In Czech Republic they use data outside OSM, but last time I checked it was entirely/mostly data released by government of that country.
Roads, buildings, hiking trails, landuse, paths shops outside Czechia are from OpenStreetMap.
Can you give example of anything that is "their own proprietary data"? So far I have not noticed anything like that outside Czechia.
https://licence.mapy.cz/?doc=mapy_pu