Polycarp is one of the elders who supposedly knew John, and he probably edited the Johannine texts; that’s who the “we know his testimony is true” at John 21:24 refers to, Polycarp & Co. Because there’s differences between Revelations and The Gospel of John, I wouldn’t argue Polycarp wrote Revelations, but he’s responsible for getting it accepted as canon alongside such men as Papias, who quoted or used Revelations in a surviving fragment (Papias of Hierapolis.).
Where Revelations first came from, hell, it could’ve been a guy high on mushrooms. It’s pretty wild.