I’m confused also by how Deadpool was in the MCU timeline and able to apply to be an Avenger. Or did his own TL have its own Avengers and a Happy Hogan who looked like MCU Happy?
He skipped Earth's. Said so onscreen. He went from his, to 616, then back again to.... forget his number.
He used Cable's device to get there, you see it on his wrist. Now, he would have had to get it modified to be able jump universes, but they don't mention how or when that happened.