It’s built from the bottom up in layers (at least traditionally, I’m not sure how the newer 3D structures for memory are constructed). The bottom layer of silicon substrate is covered in an oxide and etched selectively by photoresist and masking. Further layers are connective layers of metal lines insulated by oxide, with vias connecting metal layers to one another. The same oxide deposition, photoresist, expose, etch process is used for each layer.