I think since yinrih are arboreal, they'd have a desire to get off the ground whenever possible. This leads to them building up rather than out, with dwellings packed together and raised off the ground, with the foundation resting on stilts or pylons. The fact that their urheimat was a flood-prone river valley also means that they'd have experience with flood mitigation. Yih's 12% lower gravity would help them build taller structures earlier than equivalently advanced human cultures. As buildings cluster together, cities start to look like Midgar from FF VII but on a smaller scale, with an undercity used for travel between buildings and an upper city where people live and work. In fact, the word “underlay” as used for hyperspcace may have originally been an analogy to this urban infrastructure.