If science can't prove that we're not existing inside a simulation, or whether we have multiverses, how could we prove or disprove the existence of God?

Even if as most atheists agree the Bible is a fallacy, that still doesn't prove there isn't a God, or multiple Gods. Nor does it even prove our own existence. It just proves our abilities to contemplate and interpret the laws of the physics we appear bound inside this universe.

The only rules we can discover and prove are the ones that constrain the environment we are confined to. Anything outside this environment is opaque to us, which may include a God... or not.

See the book Faith vs Fact by Jerry Coyne for an exploration of the topic