You use your imagination. I had a tub of random parts from a bunch of old 80s and 90s sets that were since put into the blender that is a family of small children. I would build space craft. Big freighters with internal bays to hold smaller ships. Huge bases and compounds for my other toys. Various other vehicles and structures. I was basically constantly building for 10 straight years of my life. No sets. No plans. No eye strain from screens. Just pure creativity and imagination.