Actually, Tesla uses MapBox's Valhalla routing engine, with OpenStreetMaps as one of its data sources. Like most systems today, routing computations are done on a server and then sent to the vehicle. Tesla hosts its own routing servers (running Valhalla), providing the MapBox API. Correct that Tesla uses Google for visualization (Baidu in China I believe).
As for CarPlay, the only "piece" for Apple to do is test and approve, they don't have development to do. I've lead intotainment development for two automotive OEMs, both doing CarPlay, so I have real world experience with this. You buy the special chip from Apple and put it in your system, and then the programming is all on you. Apple has great documentation, and describes their testing well. They work with you during development, but unless you find a bona-fide bug, they aren't coding anything themselves. Heck, once the chip is in your infotainment system, CarPlay can work - that's how you develop and test.
And, relevant to this thread, I have a GGT on order, supposedly a 3rd week of Nov start of production date. I've been driving electric since 2011 when I bought a Tesla Roadster (which I still own and drive). I also had one of the first 2000 Model 3s, and two Model Xs. We're terminating our Model X lease early to get a Gravity.
Navigation on the Teslas has been great. At first, we ran both Apple and Google Maps to see what routes it suggested, especially when traffic was an issue, but it was, in our experience, just as good. The way it suggests charging stops on road trips is really nice, too, as it knows which chargers are busy and which aren't, plus it optimizes overall charging time. But,I'll post my own comparisons once we get our vehicle.