How 3 Phase Transformers Work

How do 3 phase transformers work, why are three phase transformers used, how do they produce 480V, 277V, 240V, 208V and 120V. What is wye delta, open delta and high leg delta. What is a pad mounted and pole mounted transformer and how is three phase electricity generated?

Edit: at 8:36 in the video, the Nameplate shows Delta-Wye not Wye-Wye as text shows