As Flutter is rapidly gaining momentum, you'll find a lot of articles and documents on the topics that we've touched in this project. 

For padding, EdgeInsets, the box model, and layouts in general, the Flutter official documentation has a fantastic article to get you started at: https://flutter.dev/docs/development/ui/layout.

For TextFields have a look at: https://flutter.dev/docs/cookbook/forms/text-input.

For use cases of DropdownButton widgets, again the official documentation has a nice page at: https://docs.flutter.io/flutter/material/DropdownButton-class.html.