Acessing shared code from native

I am new to this and one thing I have trouble of finding information about, is how to access shared code from native code. The other way around is ok, and there is multiple resources and examples about DependencyService and how to create native spesific code, and accessing it from shared code.

But the other way around? Some native event triggers something which should modify something in the shared space. I can access methods like App.Current.MainPage.DisplayAlert, but how about custom methods?