Xamarin has unfortunately muddled their branding.

Xamarin Forms is not Xamarin, it is a DSL. You do not need to use it. At its core Xamarin is c# with pinvoke to the underlying native platform implementation. You'll still need that platform knowledge but now you can share the core business logic between platforms.

To get maximum code share you'll need a MVVM framework such as ReactiveUI which is based on the Reactive Extensions and modeled on functional reactive principals check out https://github.com/reactiveui/ReactiveUI/releases/tag/7.0.0 or http://reactiveui.net