How does a logic board power up?

1. An always-on PP3V42_G3H 3.42V power rail is created directly from internal battery or a charger. This power rail powers the System Management Controller (SMC). The Mac is in “off mode” (G3) but the SMC keeps scanning the keyboard power-on button and Magsafe connector for further inputs.
2. If you connect Magsafe to the Mac, the SMC will read code from the chip on the Magsafe connector to identify the type of charger connected. The charger outputs a pre-set voltage (14.5/16.5/18.5/20) 1 second after the connection is stabilized and the charger type is identified. The green light then turns on. The charging circuit will receive power from Magsafe. A power management chip will create a PPBUS_G3H power rail, operating at 12.6V for MBP and 8.4V for MBA and Retina. The SMC reads the chip on the battery to check the battery status and therefore determine the required charging current. If the battery is empty then a high current will be applied (fast charging mode).The current will be gradually reduced when the battery is near full. The orange light turns on while battery is being charged. Once the battery is fully charged, the green light will turn on again and no current will be sent to the battery.
3. When you press the power-on button then release it, a high-low-high pulse will be sent to SMC. SMC will enable a power management chip to create a 3.3V power rail from the PPBUS_G3H rail. This rail powers up Intel’s Platform Controller Hub’s (PCH) real-time clock circuit, deep sleep power well, and suspend power well. PCH will enable the remaining power management chips to create different voltage rails (5.0V, 1.8V, 1.5V, 1.35V, 1.2V, 1.05V, 0.9V, etc. depending on hardware design) for different devices. All these power rails can be measured on the inductors (coil). Since more than 50% of faulty logic boards are related to one or more missing power rails, this information will be helpful if you decide to get it fixed by a component level repairer.
4. The memory module (RAM) will receive power first followed by the CPU, hard drive, optical drive, graphics chip, screen, sound chip, and WiFi card. DDR3 RAM will receive the 1.5V power rail (1.35V for low voltage DDR3).  Mechanical hard drives and optical drives will receive the 3.3V and 5V rails. SSD drives will receive the 3.3V rail only. CPU and GPU core power rails are dynamic depending on load.
5. All power rails report to the SMC by sending PG (power good) signals. If any one of these rails fail to send a PG signal within a pre-set time, the SMC would shut down all power rails except the always-on PP3V42_G3H 3.42V rail and PPBUS_G3H 12.8V/8.4V rail. Multiple voltage and current sensing circuits, in addition to other protective circuits are implemented in the MacBook logic board as safety measures. Malfunction of any one of these circuits can shut down all power rails to protect the MacBook. This is the reason why MacBooks are much harder to repair than other laptop brands. If all the power rails are fine, SMC will report to PCH and it will send a “reset” signal to the CPU.
6. Upon receiving the reset signal, the CPU will load BIOS/EFI codes (hardware specific codes stored in the ROM chip) to the memory module and execute POST (Power-On-Self-Test). After POST, all devices including the hard drive, optical drive, USB, sound card, WiFi and GPU will be initialized and ready to go.
7. The GPU will read the code chip on the LVDS (Low Voltage Differential Signalling) or eDP (Embedded Display Port) screen circuits for resolution setting and switches on the screen image processing logic circuit and back-light circuit. The Mac will chime at the same time as screen back-light turns on.  If you hear a chime but no image on the screen, shine a touch from the back of the screen through the Apple logo and if you can see the image, then the Mac has back-light problem.  You may have a faulty screen (LED light or cable fault) or faulty back-light circuit on the logic board. Apple puts the back-light circuit on the logic board instead of on the screen as other brand names do. This is the major reason why other screen brands cannot be used on MacBook. This design also makes MacBook particularly vulnerable to water damage.  Water can kill the 28V-50V back-light circuit instantly.
8. The CPU starts and executes BIOS/EFI’s PnP (Plug and Play) codes to allocate resources such as DMA channels, I/O port address and IRQs.
9. The CPU then executes BIOS/EFI’s input/output codes to access
   bootable devices such as hard drives, optical drives or USB drives, searching for an operating system loader. If no bootable drive is found or the bootable drive does not have a loader, a question mark will be displayed. If a loader is found, the Apple logo will be displayed. The CPU then executes the loader and loads the rest of Apple’s OSX software into your Mac. You will see a loading progress bar while OSX loads (older OS X uses a rotating gear image).
10. The loader loads the core OSX first followed by application and hardware drivers. MacBooks with graphics issues usually start going wrong at 2/3rds of the loading progress bar, corresponding to the moment the graphics driver is being loaded and executed.
11. Upon finishing loading, control is passed to OSX.