Sandwiches are layered, handheld foods that do not require any cooking post construction to eat.
Dumplings are enclosed foods that do require cooking post construction to eat. Dumplings can be further characterized by how they are cooked (baked, boiled, steamed, etc).
Maybe later I will investigate the actual history behind these classifications and the real government law about these foods, but the short version is that the internet’s pet theory of how to classify sandwiches is totally wrong.