AlmightyKfish
This Is No Longer A City.
Yeah I think that's basically the jist of it.
I think maybe there's a bit more vague knowledge of something called the incursions that was a problem, but people don't know the specifics, and they may not have actually occured outside of people's memories. I say this because we saw Maria Hill mention 'the incursion crisis' whilst talking to a bunch of people. But yeah, only a few seem to know exactly what happened.
But yeah the big thing is that in actuality the incursions were never stopped. The entire omniverse did collapse and die but then obviously then Doom, Owen and Strange saved it, then later Reed, Owen and Franklin build the new one. Which is why the Beyonders are still dead and not trying to murder everyone again, and why the incursions aren't actually happening. And maybe never did.
For instance, I'm fairly certain Iron Man/Captain America don't remember beating each other to death before the last incursion as that would make things...awkward.
But it seems many of the heavily involved people remember. Molecule Man obviously, Galactus, Black Bolt, Doom I assume by the way he laughs at the end of SW and has been following up on doing things differently. And then a few more dotted across the books who clearly remember too.
But really, the answer is whoever it's convenient for the writers. Which I guess is kinda fair enough for the sake of simplicity.
But yeah the technical how, Owen's speech in Ultimates basically said that the vast majority of lives continued from the old Omniverse into the new one, just there are some differences. The main being Miles from our standpoint I guess, whose life Owen specifically pasted out from the UU (and brought back his mother too).