What are your worldbuilding headcanons for civ (i.e. something like "the leaders are immortal/ghosts who remember everything from their pasts")?
I think the best way to explain the logic for Civ in a way that still somewhat fits with gameplay is to assume that the leaders are indeed immortal and that they have been resurrected from their previous irl lives into a different dimension than our universe. They are resurrected with all their memories of their pasts, which explains the leader agenda system (each leader’s main agenda is roughly based on something their irl self did or something the civilization they led was famous for). It also explains the special relationships between leaders who actually met, knew each other personally, or at least were aware of each other’s existence (if they didn’t actually meet) in real life.
I like to imagine there being a “central universe,” nearly identical to our own (as in, there is a planet Earth and there’s our solar system and all the galaxies in our universe) where all resurrected leaders live their daily lives and spend most of their time, except that the “central universe” follows Civ game logic and physics (everyone’s immune to dying of old age but can be killed by force or starvation, etc.), and that each time a player begins a new game, the handful of leaders that are in that game are duplicated from the “central universe” and placed into a “duplicate universe” with an Earth whose appearance is based on that player’s map settings and the duplicate universe’s logic follows whatever game settings the player decided on.
I also like to imagine the leaders having had childhoods in this alternate central universe where they interacted with other young leaders, but that’s a little harder to explain (if we assume that, as soon as Firaxis decides to add a new leader to Civ, that leader is immediately resurrected and placed into the alternate central universe, already as an adult). Instead, we can assume that all possible candidates for Civ leaders (meaning, all well-known, accomplished people who led irl historical civilizations that could possibly be picked for Civ games or have already been chosen for Civ) were resurrected into this alternate universe, all at the same time, as babies, and that, after they grow up and Firaxis releases a new Civ game, select leaders are bestowed “leader status” for that Civ game. We can also assume that the Civ central universe runs parallel to our own, and this determines the timing of “leader status” designations. For example, before the Vietnam and Kublai Khan pack was released for Civ VI, Ba Trieu existed in the Civ alternate universe, but she did not have “leader status” yet. However, upon the release of the pack, Ba Trieu gained “leader status” and could be placed into “duplicate universes” for games.
Also, the way leaders age in this alternate “central universe” goes like this: All leaders start as babies. (Their irl parents are resurrected into this universe, too.) They age like humans normally would, but once they reach the age that matches their depictions in Civ, they stop aging forever. This does mean that some leaders stop aging before others do. (As an example, compare, say, Alexander to Wilfrid Laurier. Alexander’s appearance in Civ VI seems to place him in his late 20s/early 30s, when his irl counterpart was in his prime and just a few years before he died. On the other hand, Wilfrid Laurier’s game depiction seems to place him in his late 50s or his 60s. If you check out his Wikipedia article, there is a picture of Wilfrid in 1906 that seems to be what the Civ devs based his in-game appearance on. In 1906, Wilfrid was 65 years old, right about what I estimated his age to be based on his Civ look. In my idea of the “central Civ universe,” Alexander would stop aging in his late 20s or early 30s. He couldn’t age any more, both because he would no longer look like his in-game self and because the irl Alexander the Great didn’t live past the age of 32. Wilfrid, however, would age until he was in his mid-60s, the age range when he was in office as Prime Minister of Canada, and the age he is depicted as in Civ VI.)
This all sounds really complicated, I know; I apologize if it only makes sense to me in my own head, lol.