That angel is sorta like a masthead. In this pic here, the Nauvoo is the gigantic cylinder which has yet to be closed, while Tycho Station is the much smaller cloud city lookalike to the right:
The Expanse is fairly hard as SF goes (especially by the standards of most TV sci-fi), so all the ship designs are meant to be constrained by the laws of physics. The decks in most ships - like the Cant, the Roci and the Donnager are laid out in the hull like floors in a skyscraper, so when the engines are on they provide gravity by means of acceleration.
The standard method of travelling to a destination in the show is to calculate the speed you're going to travel at, and aim for where the destination will be when you reach its orbit. The ships accelerate for half the journey, then cut engines, flip 180 degrees and decelerate the rest of the way, so they come to a halt relative to the destination upon arrival(when Miller is travelling to Eros, the PA in the ship mentions the flip over is coming up, so he was halfway to the station at the time)
The Nauvoo is designed to be a generation ship, taking centuries to travel to their destination. As a result, they can't carry enough fuel to accelerate, flip and decelerate, and they can't rely on acceleration to provide gravity. So instead the ship will be accelerated to its cruising speed, at which point most of the hull will spin around its own axis, providing rotational gravity instead.