How does zombie spawning/mutation strength actually work in Civilization VI?

Question

In the Civ VI New Frontier pass content you can play "Zombie defence mode", in which new Barbarian zombie units will spawn.
These zombies can spawn when a unit is killed by a zombie, when an undefended city is attacked by a zombie, and several turns after any unit has died there is a chance a zombie will spawn on the tile they died on.
But they seem to be spawning way faster than that in my current game. I also find that when a hostile zombie kills itself attacking a Friendly zombie (from the Twins Hero or Turn Undead), it spawns a fresh hostile zombie instantly, but if my Friendly zombie kills any unit, it does not spawn a zombie (Friendly or otherwise). I'm not sure if this is intended or a bug.
The zombies also have a "mutation strength" bonus to their Strength score, which I assume is to help them level up and stay a threat as the game progresses, but I am unsure how this is calculated, I couldn't find an explanation in the in-game Civilopedia. It's really running away in my current game, my 55 Strength units are facing +80 Mutation Strength zombies.
I understand that this is intended to be a difficult scenario, but it's weird that the zombies are acting in unexpected/unexplained ways.

Michael Campbell · Accepted Answer

As mentioned in the earlier answer, some of this is actually answered in the in-game Civilopedia:

It's Melee Strength increases with each Zombie that has been eliminated globally.
If a zombie destroys another unit, a new Zombie unit spawns in that tile the next turn.

And this is confirmed in the online Civilization Wiki
So Mutation Strength goes up by 1 every single time a Zombie unit dies, and a Zombie will spawn next turn any time a Zombie kills a unit.
Going and carefully re-watching the February 2021 Game Update Developer Livestream has answered the rest of my questions here.
In the Livestream the Devs explained a number of other ways zombies spawn:

Every Barbarian Camp initially spawns "2 or 3" Zombies in addition to it's usual Barbarian units.
Any time a unit dies (to anything, not just Zombies), there is a chance a Zombie will spawn in that tile later. The Devs say there is a "5 turn and 50 turn" chance, but that for destroyed Zombies "the length of time is even longer".
When a Zombie attacks a City Centre tile there is a "low chance" that the City population will go down by 1 and an extra Zombie will spawn in an adjacent tile.
Finally, they explained that killing a unit with a Zombie unit that is under your control (from any source) will spawn another Zombie unit that is not under your control.

So I must have been misattributing the Zombies that spawned when my Zombies killed other units (like the waves of attacking Zombies they killed on Defence) as unexplained extra Zombies because they spawned after the Barbarian turn alongside all the other hostile zombies spawning.
It also seems that the "friendly zombies create more unfriendly zombies" thing is not a bug, but is actually the intended behaviour.
No wonder my first Zombie Defence game quickly degenerated into a hopeless World of The Dead situation...

Robert Stegmann · Answer

The mutation strength increases everytime a zombie is killed.
On the Civilopedia page for the Zombie unit

Its melee Strength increases with each Zombie that has been eliminated, globally.

How does zombie spawning/mutation strength actually work in Civilization VI?

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