Note: this refers to the traditional DC Comics version and to a much lesser degree the current one, not the TV show of the same name.
One of the reasons why DC abandoned its parallel Earth model back in the 1980s was they felt it was too confusing. I admit I was a bit confused at first, when I was 6 in 1977. I think at first when I read about the Earth-2 heroes the Justice Society (Earth-2 was the world the 1940s hereoes lived at the time) I thought it was the same characters in different costumes, or something like that; regardless I didn’t get it at first. And then there were backups with the Earth-2 Green Lantern in the Green Lantern comic at the time and I misread “The Original Green Lantern in Chains” as “The Original reen Lantern Change” and thought the old version was becoming the new. But then I read a Justice League comic teaming the Justice League and Justice Society comic and I finally understood the concept of parallel Earths and never had trouble since understanding the concept; ! I think I was 7 at the time. Of course, once I understood what parallel Earths were, grasping Earth-3, Earth-S, Earth-X etc. wasn’t that difficult; it was just a case of figuring out the nature of the heroes of that Earth.
Since then DC has changed their history multiple times, removed parallel Earths, brought them back, created new worlds also called Earth-2 (one a reworked Earth-3, another pretty close to the old one, and now a radically different one only loosely based on the old Earth-2). Frequent history changes and parallel Earths that do and don’t exist according to current policy and keep changing the nature when they do exist? That's much less confusing than the old parallel Earth model!