I think the most interesting thing about circuit boards is that they're "printed" circuit boards. The pattern that will eventually be etched into the copper foil is created using a light-sensitive film. This video shows it step-by-step

Complex electronics have many layers. They print and etch a layer, glue on another layer of fiberglass, copper, and photoresist, and print the next trace layer. The final layers of solder-mask and symbols or letters are silk-screen printed - almost exactly like t-shirts.