There've been a few threads related to modding city styles. I"m not sure if anyone has succeeded in actually
adding a new city style (with a fake DLC), but I"m fairly certain that that replacing existing ones should be possible in a standard mod.
See this thread:
https://forums.civfanatics.com/threads/modding-city-art-styles.528081/
And post #6 and below in this one:
https://forums.civfanatics.com/thre...-for-wonders-buildings-citystyles-etc.519418/
How the <Culture name="Asia"> or <Culture name="America"> tags works?
Where do they transfer the reference?
My guess would be that the exact relation is hardcoded into the graphics DLL (to which we don't have access).
However, there's also the ArtDefineTag, ArtStyleType, ArtStyleSuffix, and ArtStylePrefix columns in the <Civilizations>-table which use a similar naming format (although they're not a 100% match all the time) and are likely related
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Unit graphics on the other hand are very moddable! The <Units>-table contains the UnitArtInfoCulturalVariation-column, which is only used by settlers in the base game.
Inspecting the <ArtDefine_UnitInfos>-table reveals that settlers have four entries: ART_DEF_UNIT__SETTLER, ART_DEF_UNIT__SETTLER_AFRI, ART_DEF_UNIT__SETTLER_AMER, ART_DEF_UNIT__SETTLER_ASIA
Note how the last part of these entries matches the ArtStyleSuffix in the <Civilizations>-table (e.g. _AFRI). In other words, civs that have _AFRI as their ArtStyleSuffix use the ART_DEF_UNIT__SETTLER_AFRI graphics for their settlers!
You're not limited to existing ArtStyleSuffixes. For instance, CL's Simba mod uses an ArtStyleSuffix of "_LIONKING" to make certain units look like Lions.
An important note is that units cannot
both have a UnitArtInfoCulturalVariation and UnitArtInfoEraVariation. The latter allows for different graphics based on the owner's era, which is used by things such as workers, caravans, or great people.