Alright, alright, alright, I think I’ve got it. A unified theory tying together every era of DC comics, TV, and movies into one massive narrative. We start at the beginning: Perpetua creates our multiverse with Direct-Crisis energy, Hands imprison her, she creates a Crisis to turn the Universe into a multiverse and then back again in a Bootstrap Paradox that would make Toby Whithouse proud.
Then, due to the efforts of Alex Luthor, Jr., New Earth splits off into a multiverse in its own right, consisting of 52 identical Universes. Then, Mister Mind comes along and muddles up every Universe’s timeline, making them proper alternate realities. These realities have their history altered again and again by beings like Barry Allen, Pandora, Jon Osterman, and Wally West, but there are never more than 53 Earths.
Until…
(Spoiler: Dark Crisis)
Pariah, who has helplessly watched some of these events unfold, recreates the multiverse as Krona originally accidentally made it. The number of Earths is now infinite, and cataloguing them a Sisiphysean task, so naturally Barry Allen tries to undertake it.
And he’s not the only one. In an infinite multiverse such as this one, there are inevitably infinite nerds who try cataloguing the Earths, many, if not most, referring to their own homeworld as Earth-0 or -1. Thus, many different "Earth-1"s co-exist in the same multiverse, and crossovers flourish.
(Spoiler: Batman #131-135)
An example of this is Darwin Halliday, who travels the multiverse creating Jokers where none yet existed, making them even more twisted when they do, and resurrecting them where they’ve died. One of the latter is the Joker of Earth-789, who is revived only to be defeated by Batman again, as Alexander Knox is reading when he looks up at the sky and it is… Maroon.
(Sorry, couldn’t resist)
So, the new Anti-Monitor destroys everything, only for the Oliver Queen of one of the many Earths-1 to sacrifice himself to reboot the multiverse into another new one. Now, Oliver’s Earth-1 is combined with Earth-38 and Earth-… Uh… Black Lightning… into a new “Earth-Prime”, while Earth-1 is a world where Superman carries the Kryptonian Genetic Codex, Aquaman is hot, and the Batsuit adds 80 pounds, and Earth-2 is home to a younger Batman and the JSA.
But there’s one who remembers. One who remembers everything. Everything that’s happened since there were 52 identical Earths way back in ancient 2007 is because of him, when he ate time itself and named the gods. He has watched this all knowing everything, and his plan seems like it’s… Eventually… Going to reach fruition.
Until…
Well, I guess you’ll have to watch the The Flash movie to find out what happens next! But suffice it to say, it’s all there. Every era of DC meshes together in some form as long as you allow for there to be multiple numbering systems within the same iteration of the multiverse.