Using Javascript against itself to help us PWN - Intigriti's 0521 XSS challenge

EDIT: I won again! My second Intigriti challenge win, this time in the draw for Best writeup for this very video!
If anything wasn't fully explained, it's because I tried to keep it short as I've already explained it in my old video here: • Demystifying JSFuck - ...
Another great challenge, thanks GrumpinouT!

[@mukundbhuva]

This should be awarded as best write-up

[@robbeverwilghen]

He won a €50 swag voucher for it :)

[@sahilabbasi7184]

I salute you

[@sahilabbasi7184]

Bro you're real Hacker 😍😍😘😘😘😘

[@НиколайМиронов-г3и]

Why the script has been executed? What does execute the script?