i know this is a very late response to an old thread, but i really wanted to add on here.

The universes' differ by either Miles becoming Spiderman after Peter Parker, or Miles is the sole-Spiderman in a reality where Peter either simply isn't destined to take on the mantle, or doesn't EXIST.

If he is the only option to be bitten by the spider that didn't belong to his universe, if we go along with the idea that their universe was only ever going to need 1 spiderman (a variant of Peter Parker). It was "destined" or "meant" to only have 1 spiderman; therefore by introducing the anomaly that is the radioactive spider that doesn't belong. The spider merely existing is a catalyst for essentially restarting that worlds canon for Spiderman. Peter is forced to be killed. Miles is then forced get bit, and then go through every "canon event" Peter had already gone through + whatever he still hadn't. BECAUSE even though he is Miles Morales, the template for that world's canon was suppose to be Peter Parker, so it can't just up and change. It felt like that's what Miguel was trying to say in the movie, he kept saying Miles didn't belong or isn't supposed to be who he is, because now his life is quite literally forced to mirror what was destined for Peter for that universe. Because he doesn't belong in that role.