All the universes existed from the start. Most just haven’t diverged yet. At any given moment, there are an infinite number of completely identical universes.
The universes literally split, and some quirk of quantum mechanics makes this actually possible.
They aren’t universes, they’re timelines. All the universes are in quantum superposition with each other.
There aren’t actually multiple universes. It’s just acknowledgement of the infinite possibilities. (This is how I like to think of most quantum mechanics, tbh.)