How do emoji skin-tone modifiers work?

Unicode defines five skin-tone modifiers (Fitzpatrick Types 1–2, 3, 4, 5 and 6). You append one to a base human emoji and the platform renders the combined form. So 👋 (base) + 🏾 (modifier) = 👋🏾. Platforms that don't support the modifier just show the two characters side-by-side.