A computer relies on electricity, normally from the batteries in them, like a phone, or iPad, or laptop, and desktops(PCs) use plugs. When the switch of the computer is on, there is a wire near the power button, and that sends a signal to a unit called the PSU (Power Supply Unit), which handles signals from the power button, and gives electricity to the motherboard. Within 1 or 2 seconds, the motherboard has enough electricity to turn on the computer, and tells the computer, "Hey, its time to turn on!", and sends a signal to the CPU saying, "Hey, wakey wakey! We are on!", which gives the processing a start, and starts POST (Power-On-Self-Test), or basically, the thing that checks if the computer is healthy, and then sends a video signal the monitor, saying "Hey, here's the video!" what's happening in the computer, and therefore, the monitor turns on, since it got shown the video signal, and the computer turns on.
