How does the 3.3V output work in this PSU circuit?
That's a simplified diagram how a power supply might work - not how all power supplies work.

Quote how it works from the Wikipedia article you linked :


  In the most common design this voltage is generated by shifting and transforming the pulses of the 5 V rail on an additional choke, causing the voltage to rise delayed and rectified separately into a dedicated 3.3 V rail and getting the rising idle voltage cut by a device type TL431, which behaves similar to a zener diode.