The multiverse exists because it exists outside of our plane of existence. We can't interact with a parallel universe because its timeline is shared with ours, like a mirror. We can only interact with potential universes before time catches up to us.

Just how a sphere is a circle with infinite points in 3 dimensions, our reality is a single point in an infinite possibility of realities. We can know this because we have the ability to shape our reality by interacting with it. Manipulating it before time catches up to solidify it as present, then past. We make one choice, and because of that, the opposing choice is made in a different reality.

Trying to conceptualize it within our universe doesn't make sense because it's operating on laws we cannot fathom. We are bound logically by the laws in our universe, so even trying to imagine it is like trying to teach a sloth quantum physics and expect it to understand.