Eventually people started pointing out a problem, though. Superman, Batman, Wonder Woman, Aquaman, and Green Arrow were on both the Justice Society (on Earth-Two) and the Justice League (on Earth-One). Those characters were never cancelled, and the point of both teams was to show all the most popular characters. But how could they be on both Earth-One and Earth-Two, and how come they weren't older, like the rest of the Justice Society characters were? So the logical solution was to say there were two copies of each of these characters. Technically, the originals were on Earth-Two, and technically, the "main" versions in the then-current comics were not the originals. The points at which the comics switched from the originals to the new ones was at some unspecified point in the past. They used this, though, to explain away any inconsistencies that had cropped up since their first appearances. For example, originally Superman worked at The Daily Star, and fought a red haired Luthor. At random points, they just started calling The Daily Star The Daily Planet, and Luthor was just suddenly bald (and Luthor's later origin story would show him going bald as a child). Now they said The Daily Star and ginger Luthor were on Earth-Two, and The Daily Planet and bald Luthor were on Earth-One. However, people also pointed out that this didn't make total sense, because these changes were gradual, and there were still stories with The Daily Star but bald Luthor. Therefore, DC was forced to explain that the real stories that were actually published mostly took place on Earth-Two-A, and technically, Earth-Two isn't the universe from most original comics, but a new universe that was technically only created that first time The Flash crossed over with Jay Garrick. However, this is not actually important, and there are no actual stories where Earth-Two-A is important. Earth-Two is just treated as the original universe.
DC also bought Charlton Comics, who owned characters like Captain Atom, The Question, and Blue Beetle. However, this happened shortly before DC was about to do a story called The Crisis on Infinite Earths, where they were gonna try to merge all the universes into one, because they thought having some comics on Earth-One and some on Earth-Two and one on Earth-S was just confusing. And while at it, they'd get rid of all the other universes that were just occasional locations to visit (like Earth-Three and Earth-X), even though they weren't that confusing. The Charlton Comics universe appeared in The Crisis on Infinite Earths, called Earth-Four., but in that same story, it was merged into the main universe, along with all the others. Now, this would mean it should be relatively simple, but at the same time, writer Alan Moore had an alternate idea for the Charlton characters, an edgy deconstruction of them that would result in them all being killed or ruined. DC told him he wasn't allowed to ruin all these characters they just bought, so he changed them to OC stand-ins and made Watchmen. Watchmen was not part of the DC multiverse at all, so this shouldn't be confusing. But Watchmen was uber successful, so 30 years later they would finally just give up and have it cross over with the main universe in major ways, which again made it more confusing.
Now, merging the universes (which happened in 1986, btw) necessarily resulted in history changing, because, for example, they were now saying The Justice Society and Justice League were always the same universe. So sometimes people treat this "New Earth" (what the merged universe) as a completely different universe, but technically it isn't, it's just Earth-One with the other universes pasted on top of it, which results in history being changed. History would also change several more times, like in Zero Hour (1994), Infinite Crisis (2005), and Flashpoint (2011). Technically, these are all the same universe, though.
However, with Infinite Crisis, they created a new multiverse, because they started realizing that deleting the multiverse but keeping characters originally from multiple universes actually made things more complicated, not less. But the new multiverse was technically not the same as the old universe. There were some universes based on old universes, but they were very clear to note that they were technically not the same universes. So there was a new Earth-2 (not Earth-Two), and it had guys like the Justice Society, but the actual original Justice Society from Earth-Two still lived on New Earth (originally Earth-One, now called Earth-0). There was also a new Earth-3 with a new evil version of the Justice League, a new Earth-4 based on Earth-Four, etc. After Flashpoint, when Flash broke the timeline, history was changed in all universes, some more than others. The post-Flashpoint worlds are named without hyphens, so Earth-2 and Earth 2 are actually the same universe, but they're very different, though they both have characters based on the Justice Society, though the original Justice Society from Earth-Two still lives in the main merged universe (currently called Prime-Earth, not to be confused with Earth-Prime, which is the "real world" where the writers and artists of the comics live).
On the topic of Earth-Prime, it was originally presented as the real world, and there would be stories where Superman or whoever would sometimes go to the real world and meet the people making his comics. However, during the Crisis on Infinite Earths, Earth-Prime was destroyed (or rather, merged) with all the rest. But right before it was destroyed, Superman visited one last time and met a kid named Clark Kent, named after the comic book character, who found out that he actually was from real world krypton, and gained superpowers, but right before his world was destroyed, so nobody knew about it. As Superboy (or Superboy-Prime, to denote which universe he was from), he helped save the day, but his universe was still gone, and he had no place in the history of the merged universe. So he, the original Superman and Lois Lane from Earth-Two, and the son of the good Lex Luthor from the backwards universe where the good guys are evil, all went to a "Paradise Dimension" made by Luthor Jr.-Three's powers. Years later, Superboy-Prime and Luthor Jr-Three would become villains when they tried to recreate the multiverse to bring back their old worlds, even at the expense of the current universe and everyone living there. But they did end up doing it (but not to the extent they wanted, so there were only 52 universes instead of infinite universes like before, and also not everyone died). Earth-36 was now treated as the "Real World," and eventually Superboy-Prime got his powers taken away and he was sent there, even though technically it isn't really the universe he was from, but it had a history as if it was, and the people there were copies of people he knew, and were missing him (or apparently a version of him that never really existed but was exactly the same as the real him) so he fit in there. But of course he didn't like it because now he's a supervillain and knows comics are real but doesn't like how they changed and got all edgy since he was a kid. Anyway, this means that technically, our real world was only created in 2007, and all our memories of everything before that aren't really real, but just a fake history created when Captain Marvel villain Mr. Mind, a superintelligent Venusian caterpillar (originally from Earth-S, not that that's relevant) absorbed time travel energy by hiding inside timecop Booster Gold's robot-sidekick Skeets, and then used that energy to evolve into his moth form, which was bigger than the whole multiverse, and then he fed by biting off parts of universes, which changed their histories to not all be copies of the main DCU. So all of our memories and history from before that happened, in 2007, are fake.