If you are designing an add-on board (HAT), the reduced schematic + GPIO documentation is sufficient. If you are trying to repair a Pi 4, the reduced schematic plus multimeter probing usually works.
🛠️ Deep Dive: Raspberry Pi 4 Model B Reduced Schematic & Hardware Overview Raspberry Pi 4 Model B Full Schematic
Unlike many consumer electronics, the Raspberry Pi Foundation is relatively open. While the board layout (PCB files) is not fully open source (they are proprietary to protect commercial interests), under a limited license. If you are designing an add-on board (HAT),