You start out on the surface on a mine shaft, faced with several deposits of valuable ores. You begin building your basic mining rig, harvesting the resources and transporting them via conveyor belts to your storage center. You gather more resources, invest into research & development, discover new techniques and machinery. Your factory grows, your processing chains grow longer, you need to expand.
So you dig down deeper into the earth, discovering new materials and opportunities. All the while your factory must stay humming and busy, so you swap back and forth, adjusting conveyor paths and bottlenecks in the system.