The world node setup is for the world background, not the buildings. Prove a screenshot of the building material setup by changing the pulldown in the Shaders Editor from "World" to "Object."
Right now, I'd say you deleted almost all the nodes for the materials for your city model, and all you've got is the empty material output. The solid view has its own color assignments that are independent of node setups or even the render engine, so that's why that color is maintained in solid view.
Further node materials explanation: A general material node flow goes:
- Texture coordinates (defaults to generated or UV if not present). Outputs a vector into...
- Texture. Images are the usual, but can also be procedural textures like "noise" or "checker." Outputs color and alpha into...
- Shader. Principled BSDF is the all-purpose tune-to-need shader, but Diffuse BSDF is a good choice for a simple surface. Outputs BSDF instructions into...
- Material Output.
While piping the color directly into "Material Output" is possible, it only works as of 2.80 where Blender tries to work around one output being piped into a different output type. Check out the colors and labels on the node inputs and outputs. In this case, blender is using the color like it is being fed into an "Emission" shader node (the simplest of all shaders, and Cycle's specialty because it can be used in Cycles to create truly glowing meshes).
The world material is similar to object materials, except you usually don't need coordinates for that, and you want to use "Environment Texture" so it will map properly. The world material makes a lit background as you can see with your trees.
Volume materials fill things with smoky looking stuff. Use it on the world for a universal fog, but be warned that in Cycles it will block out the sun and any world background you might have, because it extends infinitely into space.
"Emission" shader can work in volumes, which is handy for making space rays :-D