I don't know either but here is the answer from Wikipedia: "The main provider of map data is TomTom, but data is also supplied by Automotive Navigation Data, Getchee, Hexagon AB, IGN, Increment P, Intermap Technologies, LeadDog, MDA Information Systems, OpenStreetMap, and Waze."[1]
[1] https://en.wikipedia.org/wiki/Apple_Maps