Do I need to use a second project in Xamarin if I'm only writing for android?

Question

Xamarin

Good day.
Started learning Xamarin.Forms, started writing an application for Android. Faced such questions, is it necessary to create business logic in the second project (which refers specifically to Android'u)? Or is it possible to write everything in the first project (both the behavior of the models and the view model)?
In other words: is it necessary to write code like this ? (Given that I write only for Android)