This is a quick sketch in Plasticity of a stereotypical dense urban center. Buildings sit on pylons above the ground, and there are pedestrian walkways between buildings on the level of the real first floor of the building.
There are a few factors that lead to this design choice. First, yinrih are arboreal animals and prefer to be high up if they can help it, so most buildings from houses to skyscrapers are constructed on stilts or pylons making the “first floor” above the ground and leaving an open space called an underlay underneath.
Second, The greatest rule of yinrih urban planning is that vehicles and pedestrians do not occupy the same space. The vast connected underlays of densely populated areas are used for vehicle traffic, with foot traffic accommodated above. The subspace used by ansibles and mass routers for FTL communication and travel is named after this feature of the urban landscape.
Underlays are also home to electrical and plumbing infrastructure as well as bums and junkies.