What is the equivalent of a Grid?

The closest equivalent of a Grid would be a GridView.
This is much more powerful than what you are used to in Xamarin.Forms.
A GridView provides automatic scrolling when the
content exceeds its viewable space.

You might have used a Grid in Xamarin.Forms
to implement widgets that overlay other widgets.
In Flutter, you accomplish this with the Stack widget.

This sample creates two icons that overlap each other.

What is the equivalent of a ScrollView?

In Xamarin.Forms, a ScrollView wraps around a VisualElement,
and if the content is larger than the device screen, it scrolls.

In Flutter, the closest match is the SingleChildScrollView widget.
You simply fill the Widget with the content that you want to be scrollable.