After reading through this very interesting thread, I was wondering if I could actually name "The Big 5" publishers. I managed to get three correct, and one half correct (I didn't realize that "Penguin" went with "Random House"). During my search for the answer, I found a pretty neat chart of who-owns-who-owns-who:
https://almossawi.com/big-five-publishers/
Most of the book publishers I remember from "back in the day" (Ballantine, Bantam, Del Rey, etc.) are now a part of the Random House leg of the big 5 Penguin Random House publisher. This probably has a lot to do with the type of books I was reading back then (SciFi).
This is mostly useless trivia for me, but at least it helps me see "who/what goes where" in some of the discussions that pop up on this forum.