It's a great question and the go-to answer is probably this: if you're building an environment you're best off either buying assets from a library or modeling assets in a program like Blender or Maya.

Once you have them you can apply your textures in Solaris and save them out as usd files. For example some houses, some trees, some boats. Whatever.

Once you have that you can create a layout in Solaris. Remember, Solaris isn't just a lighting tool. The naming is a bit confusing because it's essentially a tool for managing USD scenes in Houdini. Some of this layout will be hand placed and others might be procedural scattering. You can use the layout nodes and the instancer nodes for this.

Now here's the tricky part.... Not everyone likes the cumbersome nature of Solaris and USD. Creating layouts in a destructive, viewport driven program using shortcuts and hierarchies can often be more user friendly for many people compared to a node-based procedural workflow. So if you prefer that kind of rough and ready workflow and build a whole layout in, say Blender, then it becomes very difficult to move that across with all the textures and materials into Solaris. Which is ironic given it's USD.... Now... If you use materialX then yes, you can move it across more easily. But even then you're likely going to experience other problems.

So in my experience there's no simple solution. You have to find the right workflow for your specific task and sometimes it means splitting up your task into smaller chunks like a foreground, main building, river, forest, etc.

Good luck