What is an integration test in Flutter, and how does it genuinely differ from a unit test or a widget test in terms of exactly what it actually verifies?

An integration test verifies that several genuinely separate different parts of your app all correctly and properly work together as one single cohesive whole, running your actual full app on a genuine real device or emulator and simulating actual real genuine user interactions across an entire multi step flow, such as properly logging in and then successfully completing an entire checkout process, whereas a unit test verifies just one single small isolated piece of logic and a widget test verifies one single specific widget in isolation, meaning an integration test provides the genuinely highest overall confidence that your app actually genuinely works correctly end to end, though it also genuinely runs considerably slower than either of those other two testing approaches.

testWidgets('complete checkout flow', (tester) async {
  await tester.pumpWidget(MyApp());
  await tester.tap(find.text('Add to Cart'));
  await tester.pumpAndSettle();
  expect(find.text('Checkout'), findsOneWidget);
});