From what I could tell and based on prop makers of the soft parts, the back plate is the same as the mounting mechanism for the new jetpack.
The vest and or the jumpsuit/shirt, have straps that go around the slots at the shoulder and velcro on the front and from the center back, coming through the slits on the bottom of the back plate and velcro on the sides of the vest/jumpsuit/shirt. It's the strapping system the soft parts have attached to that holds the back armor which holds the pack, there is no metal frame as the original Fett was or the alternate version used in the Mandalorian and Fett in the same series.
I believe the straps goes attached to the vest and come through slits on the back armor.
