Other useful widgets and classes for creating a responsive UI:

AspectRatio

CustomSingleChildLayout

CustomMultiChildLayout

FittedBox

FractionallySizedBox

LayoutBuilder

MediaQuery

MediaQueryData

OrientationBuilder

Other resources

For more information, here are a few resources,
including contributions from the Flutter community:

Developing for Multiple Screen Sizes and Orientations in
Flutter by Deven Joshi

Build Responsive UIs in Flutter by Raouf Rahiche

Making Cross-platform Flutter Landing Page Responsive
by Priyanka Tyagi

How to make flutter app responsive according to different screen
size?, a question on StackOverflow

Creating an adaptive Flutter app