Me trying my best to explain yield* syntax of dart.

yield.dart

Iterable<int> inner() sync* {
  for (final i in [4, 5, 6]) {
    yield i;
  }
}

Iterable<int> outer() sync* {
  for (final i in [1, 2, 3]) {
    yield i;
  }
  yield* inner();
}

void main() {
  for (final i in outer()) {
    print(i);
  }
}