It's really flatMap/map from the link:

  List<Integer> s1 = Stream.of(1, 2);
  Supplier<Stream<String>> s2 = ()->Stream.of("A", "B");

  s1.flatMap(v1 -> s2.get()
                   .map(v2 -> tuple(v1, v2)))
    .forEach(System.out::println);

That's just a nested loop.