Question about incursions in the MCU

So for Doomsday, the big thing that we all know is going on…is incursions.

It’s when one universe and another universe collide.

What I don’t understand, though, is…what do our heroes fighting each other have to do with it?

For example, there’s all this buzz and talk about the X-Men fighting Tobey’s Spider-Man to “save their universes”…but how does that even work? Aren’t the universes colliding anyway? I don’t think the universes being destroyed are incumbent on if the heroes within those universes win their fights, right?

How would the X-men or Tobey’s Spider-Man winning “save” their universe? Or are they simply trying to not die in the incursion? Because if they’re simply trying to not die, then they just need to agree not to fight one another and survive together, right?

Or is that just a lie that Doom tells everyone? “Your world will die unless you kill the other guys”