How do I....

[Atilius]

The problem is I am not sure how to enable other faction's units for each faction, I imagine that it will take a lot of work.

I'm assuming that you already have modified export_desrc_buildings.txt to allow your faction to build the other faction's units.

What you need to do is add the faction's name to the "ownership" line in export_descr_units.txt. The easy thing here is to just put "all" instead of a long list of faction or culture names.

Next, you need to unpack the pack directory - see
https://forums.totalwar.org/vb/showthread.php?t=40313 - and move unit cards files and unit description files to the UI folder, which you'll need to create. The details can be found in this excellent tutorial : https://forums.totalwar.org/vb/showthread.php?t=51750.

For each unit which you are going to allow a new faction to use, you must find it's entry in descr_model_battle.txt and add a texture and model_sprite line for the new faction. The above tutorial also covers this.

If you want to allow recruitment of many units by different factions this is a lot of work. You may want to take it slow. You may also need to beware of the 32 unit limit for the number of unit types recruitabkle in a settlement. See https://forums.totalwar.org/vb/showthread.php?t=42905.

Good luck.