Multitype changes the form and type of a Pokémon with this Ability depending on the Plate attached. The item-removing effects of Thief, Covet, Trick, Switcheroo, and Knock Off will not work on a Pokémon with Multitype, regardless of what item it is holding.
When a Pokémon transforms into Arceus, it becomes the same form and type as the target Arceus. If the transformed Pokémon is holding a Plate that is different from the target's held Plate, it then immediately changes to the form and type corresponding to that Plate; if it is not holding a Plate, it immediately changes to Normal Arceus (unless it transformed into Normal Arceus).
The non-white parts of Arceus's body glow white. It then changes its color freely to match that of the type it is changing into. A yellow ripple then forms throughout its body when it wants to change back to normal and its body goes back to its original colors.