This way of generating UI is different from Xamarin.Forms,
but there are many benefits to this approach.

Views

What is the equivalent of a Page or Element in Flutter?

How is react-style, or declarative, programming different from the
  traditional imperative style?
  For a comparison, see Introduction to declarative UI.

In Flutter, almost everything is a widget.
A Page, called a Route in Flutter, is a widget.
Buttons, progress bars, and animation controllers are all widgets.
When building a route, you create a widget tree.

Flutter includes the Material Components library.
These are widgets that implement the Material Design guidelines.
Material Design is a flexible design system
optimized for all platforms, including iOS.

But Flutter is flexible and expressive enough
to implement any design language.
For example, on iOS, you can use the Cupertino widgets
to produce an interface that looks like Apple’s iOS design language.