Hi
@MusashiX 96 and welcome to CFC!
Just based on your title and original question, I wanted to point out a few big-picture things:
Civ3 is great for modding thanks to its scenario editor, data-driven rules, and relatively accessible assets. As such it has a huge library of custom content, maybe the biggest in the series, and many big mods with lots of that new content.
however...
The flip side of approachable modding is that it's not all that powerful. It's easy to swap out content, but pretty much limited to that. We've been very clever with our tools over the years, but there are hard barriers. The game was made with a lot of hard-coded limits and assumptions, and predates things like SDKs and DLL loading that modern gamers expect as "mods". Civ3 is also a long-dead game in legal limbo, and there is no hope that the developers will ever attend to it again.
however...
As already mentioned, C3X and the editor by Quintillus allow us to do some things that the game was not intended to do. C3X is more along the lines of a modern mod that actually changes the behavior of the game. The concept of a larger library of civs from which we can extract a playable BIQ is intriguing, and something I had thought about before but didn't realize we may already be halfway there. (For the record
@Quintillus I'd be very interested in that for my next mod) But even with these powerful tools, we are still quite limited in many ways. The structure of the game makes it impractical to even hack many things, particularly the max/number of things, despite those often being the most wished for. We can't increase the number of civs, culture groups, eras, buildings or terrain types, cities, map dimensions, and more.
however...
Frustrated by those limitations, we've had several attempts to recreate the whole game or something very much like it ourselves. None have panned out so far, but I am currently leading a promising team effort to build such a game in the Godot Engine. It's still in the early stages of development, but "C7" aims to provide full mod compatibility with as many civs as you want and lots more.