Is Pokemon GO programmed using cross-platform technologies or is it natively coded to both Android in Java and in Swift for iOS?

Probably cross platform. The lab that wrote the app published Ingress too. I just don't thnk that they coded 4 apps in total (for iOS 2 and for Android 2). There all frameworks that can be used for A LOT of platforms (like libgdx for example). There might be a possibilty to write some "template code" that gets compiled to an iOX app and Android app. Not sure here tho...

Xamarin can be used for bth platforms. Just found another post aking for that

They probably used Unity. I doubt they could make a 3D game with Xamarin

The game on iOS has a "made with unity" splash screen

it's made with unity, plus some custom java/c++ code. But it can be made 100 in unity or any other 3d game engine.