Viorica A.

Saitama's whole jist is that no matter who he fights he always becomes stronger. That's his "power", his literally limit breaker. Vegeta, being all prideful, would try to toy with Saitama. But Saitama would quickly overpower him because of his "power" and Vegeta would lose due to his own pride.

Itadori ate the Death Paintings for blood manipulation, Sukuna is Itadori's biological uncle, so he got the shrine from there (plus having been Sukuna's vessal for a while may have imprinted it onto him). Gojo taught Itadori the basics of a domain expansion, so he just followed his teachings.