Thanks! Niantec started as a startup within Google's startup incubator and thus used Google Maps in the very beginning. However, they got 'spun out' and when the price became to expensive, they switched to OSM anno 2017.

(I'm not sure about the details, but I think the spawn points were OSM based long before the map rendering was OSM based to). In either case, read their Wikipedia page for more details (https://en.wikipedia.org/wiki/Niantic,_Inc.) and our OSM-wikipage (https://wiki.openstreetmap.org/wiki/Pok%C3%A9mon_Go)