They keep the air in by a technology called "gravity plates." Basically they are like monopole magnets: they exert a field in a certain direction.

Walking on a gravity plate would be akin to walking on sheet metal with magnetic boots (but better).

You can mount a gravity plate anywhere. Mount one (upside down) on the top of your car, and your car will fall up into the air. Mount one on the front of your car, and your car will fall forward. Whammo, flying car. Of course you can turn the gravplates on & off like electromagnets. Turn on the gravplate on the back bumper, and your flying car slows down.

The entire dayside of Ceres is covered (under the soil, of course) with gravplates at 1g. Much of the nightside is, too -- all the cities and roads and industrial areas -- but not all.

Oh, you mean the air on the wings. Heh, I just realized your meaning. Same answer, gravplates. But the concern about air applies to the planet itself, too, because its mass is so meager that it could not retain atmosphere without some technological help.

This arrangement also negates any worry that the air on the wings will "run downhill" to the planet, because center of gravity becomes a meaningless concept.

Much of this explanation can be gleaned from the story (chapters 31 and 32).

Thanks for the kudos!

The Solar System map is still very much "not abandoned"