|Computers have a power controller. It decides whether to let power stay on, let the CPU execute, let the BIOS be read, how to shutdown a computer, etc. To know what that controller is doing and why means a digital multimeter. A tool that costs less than a good hammer. And that is sold even in most hardware stores. You cannot see anything from a pattern or by inspection. You can only 'see' with the meter.|
In your case, a meter on a 20 VDC scale must measure six wires from power supply to motherboard. The purple, gray, and green wires are touched with a meter probe both before and as the power switch is pressed. Read and post those three digit numbers. Do same for any one red, orange, and yellow wires only when and after the power switch is pressed.
What results is an answer without any speculation. Without any more 'it could be this' or 'might be that'.