A question about the upcoming pokemon sets?

How will pokemon like Deoxys work? Who need items to forme change? Will this be like the three in one series.

How will pattern mons work?

Floette has six separate forms. Vivillon has 20 known patterns.

How will the Unowns work? Unown has 28 different forms. Will the Pokémon be treated like the 3 in one series or like # 41839?