What's the top rectangle circuitry is for with negative voltage?
Note that U1 and U2 require negative voltage so their outputs can swing to 0 V.
... and whats is RL1,RL2?
Relays 1 and 2 are used to select the lowest transformer voltage that will supply the load. This keeps the power dissipation in the output transistor, VT low.
Why there's two resistor (R15-R20) in parallel? (to increase wattage?)
Most likely or to create a 22.35 Ω resistor (but it's unlikely to require such precision).
What R46 and C8 is doing there?
C8 is the main smoothing capacitor. R46 will discharge it when the PSU is switched off. You can calculate the discharge time. It will have discharged by 95% at \$ t = 3\tau = 3RC \$ and by 99% at \$ t = 5\tau = 5RC \$
Why there's a Zener diode between inputs of the op-amps?
Probably to prevent over-voltage due to capacitor storage after power-down.