To get started, use nix develop.
The flake adds flutter to your dev environment and provides a nice CLI tool for easily starting up an emulator: flutter-dev. flutter-dev starts an emulator, sets up a trap to close it when finished, and runs your flutter app in interactive debug mode in the emulator. You can also just set up your own emulator and run flutter run.
You must already have adb and kvm or another VM system enabled in your configuration.nix