Originally Posted by
Bored at 3:00AM
They happened on all of those Earths, which all co-exist.
Doomsday Clock #10 lays it out pretty well, I think.
Essentially, there was one Earth that sprung into existence the moment Superman first appeared in April of 1938, which is the creation of what would become the entire DC Universe, and Multiverse, all of which exists in the overarching Metaverse. Since that moment, new Earths have sprung up as a result of Superman's unique position as the originator of this existence--and the entire genre of superheroes.
All events, irregardless of what Earth they've been living on at that particular moment, have occurred within that larger Metaverse and the characters we've been following since the beginning are the same Superman, Lois, Batman & Robin, Wonder Woman, and the rest, but their lives have been in a constant state of change due to the shifting temporal nature of their reality--which we call comic book time.
Superman is Superman, no matter what designation the Earth he's living on. He was Siegel & Shuster's rough n' tumble social crusader, and he was also the patriotic paragon of the war years, then he was Superboy, which altered not only his entire history, but the history of the entire world, splitting into two. From each choice he made, an infinite number of new realities were born.