Worlds Will Die.
And The DC Universe Will Never Be The Same."
That was the advertisement leading up to DC's 50th Anniversary celebration 12 issue mini-series, which started with Crisis On Infinite Earths #1 (April, 1985), in the story begun by Marv Wolfman, George Perez, and Dick Giordano (with plot assists by Len Wein and Robert Greenberger).
Monitor and Lyla (who became the Harbinger in this issue) in New Teen Titans, readers saw the destruction of the Crime Syndicate of America's Earth-3 with Pariah watching; only the child, Alexander Luthor Jr., the son of Alexander Luthor and Lois Lane Luthor, surviving.
Flash Barry Allen first met Flash Jay Garrick back in the day, leading to annual team-ups of the Justice League of America and Justice Society of America in the 1960s, 1970s and 1980s!
Hopefully, these links and articles will help expand your mind and give you some of the information you need to help you navigate the Crisis On Infinite Earths, from DC's two Supermen, Anthro, the Wild West, the World War II Losers and All-Star Squadron, the Outsiders, Infinity Inc., the Legion of Super-Heroes and more!