How do I draw/paint on the screen?

Xamarin.Forms never had a built-in way to draw directly on the screen.
Many would use SkiaSharp, if they needed a custom image drawn. In Flutter,
you have direct access to the Skia Canvas and can easily draw on screen.

Flutter has two classes that help you draw to the canvas: CustomPaint
and CustomPainter, the latter of which implements your algorithm to draw to
the canvas.

To learn how to implement a signature painter in Flutter,
see Collin’s answer on StackOverflow.

Where is the widget’s opacity?

On Xamarin.Forms, all VisualElements have an Opacity.
In Flutter, you need to wrap a widget in an
Opacity widget to accomplish this.

How do I build custom widgets?

In Xamarin.Forms, you typically subclass VisualElement,
or use a pre-existing VisualElement, to override and
implement methods that achieve the desired behavior.