How is U1 powered in this schematic
From the datasheet you linked to:


  Vcc: This pin is connected to an external capacitor of typically 10 mF. The natural ripple superimposed on the VCC participates to the frequency jittering. For improved standby performance, an auxiliary VCC can be connected to Pin 1. The VCC also includes an active shunt which serves as an opto fail−safe protection.


So it doesn't need a separate supply to that pin - it gets the supply internally and the capacitor is used to stabilize it.

But where does it come from?  Well, the internal block diagram in the same datasheet shows that quite plainly:



Yes, it gets it from the drain of the internal switching MOSFET, which is presented on pin 3 of the SOT-223 packaged device.