Access to preapocalyptic Weapons shapes the Deciv early game. Depending on who you're playing, and where you start, you might not have preapocalyptic Weapons available, so you might be forced to field an army with only iron weapons. But, once you do manage to lay your hands on a cache of preapocalyptic Weapons, you can immediately re-arm your units (to the extent you can afford it).
Early-game Deciv features the basic units with guns but no horses, horses but no guns, both horses and guns, or neither horses nor guns.
Spearman -> Militia
V V
Horseman -> Hussar
Archer -> Gunman
V V
Skirmisher -> Dragoon
The Archer and the Skirmisher are identical except that the Skirmisher is mounted, with the mounted bonuses and penalties (faster, but less effective against enemy Spearmen and cities, and unable to fortify). Same for the Gunman and the Dragoon, and so forth. (The Spearman and Horseman don't quite fit the pattern, since the Spearman gets a bonus against mounted units, but, well, yeah, you can't dig in against a cavalry charge when you're using the spears overhand from horseback.)
If you do not start near horses, but later acquire horses, this branch allows you to upgrade your units the same way you can upgrade them when you acquire Weapons.
(You might not always want to mount all your units, but this way you can.)
Similarly, Workers can be upgraded to Mounted Workers in addition to Mechanized Workers.
One minor note: this branch leaves survivor camps the same, that is, a unit can find weapons in a survivor camp but can never find horses in a survivor camp.
Access to preapocalyptic Weapons shapes the Deciv early game. Depending on who you're playing, and where you start, you might not have preapocalyptic Weapons available, so you might be forced to field an army with only iron weapons. But, once you do manage to lay your hands on a cache of preapocalyptic Weapons, you can immediately re-arm your units (to the extent you can afford it).
Early-game Deciv features the basic units with guns but no horses, horses but no guns, both horses and guns, or neither horses nor guns.
The Archer and the Skirmisher are identical except that the Skirmisher is mounted, with the mounted bonuses and penalties (faster, but less effective against enemy Spearmen and cities, and unable to fortify). Same for the Gunman and the Dragoon, and so forth. (The Spearman and Horseman don't quite fit the pattern, since the Spearman gets a bonus against mounted units, but, well, yeah, you can't dig in against a cavalry charge when you're using the spears overhand from horseback.)
If you do not start near horses, but later acquire horses, this branch allows you to upgrade your units the same way you can upgrade them when you acquire Weapons.
(You might not always want to mount all your units, but this way you can.)
Similarly, Workers can be upgraded to Mounted Workers in addition to Mechanized Workers.
One minor note: this branch leaves survivor camps the same, that is, a unit can find weapons in a survivor camp but can never find horses in a survivor camp.
It is possible to make survivor camps randomly choose one of several upgrades, as seen in https://github.com/carriontrooper/Alpha-Frontier/blob/main/jsons/Units.json#L704, but Unciv does not currently have the ability to display a different notification based on which upgrade is randomly selected. We can only specify one notification string, and it's https://github.com/SpacedOutChicken/Deciv-2/blob/main/jsons/Ruins.json#L56
Thus, this branch leaves survivor camps to match the notification.