If we station ourselves outside of the quantum system, then the answer is that each reality puts a complex number on every outcome, which get summed into a sing

If we station ourselves outside of the quantum system, then the answer is that each reality puts a complex number on every outcome, which get summed into a single complex number. Then, the magnitude squared of that sum becomes the probability with which we will observe that outcome. If we put ourselves inside the system then saying anything about it comes with a lot of implied philosophy about what "we" are.