One advantage of QM is that from outside the universe you still have some degrees of freedom : you can modify the phases without impacting the observable of the world.
You can make a pretty strong local change like flipping a coin, at any point in space-time without impacting the rest of the phases elsewhere too much, because the farther away in time and space from your modification, the more decoherent the phases would be and therefore their impact would be minimum.
And because the QM equations are time-reversible, you can propagate them backwards through time up to the origin of time. And you now have a new almost identical initial configuration, which when simulated forward will give rise to a universe that behave the same way until the time of the modification but then bifurcate.
You can make a pretty strong local change like flipping a coin, at any point in space-time without impacting the rest of the phases elsewhere too much, because the farther away in time and space from your modification, the more decoherent the phases would be and therefore their impact would be minimum.
And because the QM equations are time-reversible, you can propagate them backwards through time up to the origin of time. And you now have a new almost identical initial configuration, which when simulated forward will give rise to a universe that behave the same way until the time of the modification but then bifurcate.