Most electrical/electronic items (radios, TVs, washing machines, ...) used to come with a schematic, or at least a block diagram, inside the device. Similarly, published integrated circuit data sheets contained enough information to understand how the chip worked to a low-level. This changed around the 80s from memory? There wasn't a word for "open source" in electronics, as it was just the unnamed default. It was all paper based, as it predated wide-spread online distribution.