On a chip, there's really no difference between the source and drain. The source/drain can be connected to ground, another transistor or power as needed. Look

On a chip, there's really no difference between the source and drain. The source/drain can be connected to ground, another transistor or power as needed. Look at the diagram of an inverter in the article, and it should make it clearer how gates are constructed. A NOR gate is similar to the inverter, but with two input transistors in parallel, so either one can pull the output low. A NAND gate has two input transistors in series, so they must both be on to pull the output low. There's a single metal layer in the 8087 (but many metal layers in modern chips). The transistor can be wired however works best for the layout, with polysilicon or metal. If a transistor's source is connected to a neighboring drain, the silicon regions can just be merged and no wiring is needed.