The large orange structure is a Flanar balloon. The ring and cabling hanging from the underside are part of the extraction rig mining gases deeper in the atmosphere. The weight of the extraction infrastructure helps keep the city stable.
Though the city is open to the air, the individual buildings must be airtight. The gases deeper down are poisonous. Cyclonic storms sometimes cause them to upwell into the layer of breathable air where the cities float, and residents must shelter inside until the storm passes.
Because Welkinstead is a hot Neptune at the same orbital distance as Yih, the cities enjoy the same amount of sunlight that the homeworld receives.
Given that the air outside can behind poisonous, is there an enclosed way of moving between buildings?
Also, how does that affect the outside built environment? Can plants survive outside? Can some animals live outside and are they considered pests?
There's a system of tunnels between the buildings. Most residential areas are built into the foundation rather than being separate buildings. The plants are probably fake, as they can't afford the weight that water and soil would add.