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The PSU stabilizes and outputs the main voltages:

Does the green wire on the PSU drop to 0V when you hit the button? If not, the SIO isn't telling the PSU to start.

Provides the raw 3.3V, 5V, 12V, and -12V rails. Step-by-Step Desktop Power-Up Sequence

As soon as the PSU is plugged in, it sends 5V standby voltage to the Super I/O (SIO) chip and the Southbridge/PCH Reset Signal (RSMRST): The SIO sends a Resume Reset