You don’t know how, your thinking it might be a secret experiment where your being observed, but one day you found some ear pieces on your doorstep. You found out that when you put them on and you name a specific universe you wanna travel to and press the button on the earpiece, and then just press the button on it again to go back to your previous universe.

Name any kind of multi-verse and you can add a story to that too if you want