Forked Android with a Metro launcher and maybe they skinned the rest of the UI. I think I remember WhatsApp doing work to support them while I was there (anything Nokia made and didn't cancel before launching would have and sell enough units to matter), but I don't remember how hard we had to work; I'm guessing it didn't have google play services, but I don't remember if it had its own push services or if it just used long connections. I think maps/locations was weird because Nokia didn't hook the Google apis, so that was a bit tricky. I don't know if it could use the website distributed apk or if Nokia had an appstore, etc.