The actual supply takes +20 Volt Dc through the dioda bridge or filter part. This can be given to pins 11 and 12 on ua723, and also for the collector 2N3055 series-pass transistor. All the output voltage will be sampled through R1 and R2, giving around 7 Volt relating to ground at pin 4. The reference terminal at pin 6 is linked right to pin 5, the noninverting input of the error amplifier.
For nice trimming the output voltage, a potensiometer might be added among R1 and R2. A 100-pF capacitor right from pin 13 to pin 4 furnishes gain compensation for amplifier. ( Texas Instrument, Linear and Interface Circuit Applications, Vol. 1 )