I’m stuck on how he does that. I know that Golden Wind Requiem reverts any attack to zero, erasing the effect but keeping the cause, though I’m confused on if this is the cause of Requiem, or if it's a combination of both Diavolo's and Giorno's Stands.
The Stand that you're referring to is Golden Experience, not Golden Wind, that's the English name.
GER has a few abilities - life-giving from regular GE, strength, speed, etc. - but its main one is called Return to Zero. Basically, it nullifies any form of attack at Giorno and resets everything to before the attack. If you die from GER's punches, it nullifies your death over and over, infinitely in a deferent reality. This is poetic considering how many times Diavolo used his own Stand to manipulate reality.
Also, I have a theory that evolving your Stand to Requiem gives you anything you want at that moment. Giorno wanted to kill Diavolo, and so he did, infinitely...