You're all thinking 3 dimensionaly. In a new (multi-) universe created here, when the Paragons are returned to their time it's a new history for the existing worlds. There was no Earth-1, Earth-2, Earth-38, etc... This is how the universe always was since the dawn of (Oliver) time. The Creator (Oliver) made it so all the Paragons will be born/created on one Earth (Prime).
Correction -- the closing montage showed that Earth-2 (home of
Stargirl), Earth-96 (home of the Paragon Superman), and the others still exist as distinct realities (and obviously the DCEU,
Lucifer, etc. will remain distinct as well). The only merged worlds were Earth-1, Earth-38, and Black Lightning's Earth. So all of the Paragons
except Superman-96 are now native to Earth-Prime.
And yes, we all know that the history has been rewritten so that they've always been one world. That much was explained clearly in Part 5, and of course in the original comics story. But that's how ordinary citizens experience it. The main characters have had their memories restored by J'onn, so the former, separate histories are still going to be
narratively relevant for the characters who remember them.
I also disagree with the assumption that Oliver
made them one Earth. I doubt he had any such intention. His goal was to restore the Multiverse, all of it. Deliberately merging the worlds wouldn't have saved the "infinitude" of worlds, because it would've wiped out all but one of the versions of each individual with doppelgangers. (In the comic, it was a last-ditch desperation move to save what little was left. By contrast, Oliver was recreating what had already been destroyed -- more like
Zero Hour than
Crisis, actually.) The merging of the three CW Earths happened by accident, apparently as a side effect of the fact that all seven Paragons (including the ringer Lex) came from Earth-1 and Earth-38 (with BL being the anomaly, perhaps a consequence of his interaction with the antimatter cannon).