The universe is so much bigger than you realize.
Why it is hard sci-fi
Soft on the physics, hard on the rule. The films multiverse rule is that any branch can be accessed by performing an action with very low prior probability, called verse jumping. The rule is followed strictly. Every jump in the film is a low-probability action. The film is a multiverse comedy with an internally consistent transit method, and that is the science.
Science inside it
Many-worlds branching with a probability-weighted jump model, identity persistence across branches, and a cognitive overload failure mode for users who experience too many parallel selves at once. The film treats the verse-jumping rule as an experimental protocol with a written procedure.