The edges of the NIC cubes form walls, and the walls that are perpendicular to the ground bear the weight and keep the structure up. It is kind of hard to explain, but think about taking a carboard box with no top and turning it upside down. Now, if you put something on top of the overturned box, the sides of the box support that weight. You have to zip tie or secure the panels together pretty tightly so that they reinforce each other and don’t shake too much.
The dowels/1x2s that you see are to support floors. If you zip tie four panels together in a square, then pick it up by the edges, you’ll see that it sags in the middle. The zip ties do not offer enough support to allow direct weight on a conneciton like this, so the dowels are used to offer support to the middle part of the floor. 1 1/2 inch dowels or 1×2 lumber will snap right into the perpendicular walls of the condo, and if you slide them across the middle of the floor, they become braces, and no sagging.