While there are Gradle files under the android folder in your
Flutter project, only use these if you are adding native
dependencies needed for per-platform integration.
In general, use pubspec.yaml to declare
external dependencies to use in Flutter.
A good place to find Flutter packages is pub.dev.

Activities and fragments

What are the equivalent of activities and fragments in Flutter?

In Android, an Activity represents a single focused thing the user can do.
A Fragment represents a behavior or a portion of user interface.
Fragments are a way to modularize your code, compose sophisticated
user interfaces for larger screens, and help scale your application UI.
In Flutter, both of these concepts fall under the umbrella of Widgets.

To learn more about the UI for building Activities and Fragements,
see the community-contributed Medium article,
Flutter for Android Developers: How to design Activity UI in Flutter.