@jacks0n I saw those files in the main Civ VI Assets folder, but I don't see anything in this mod to enhance the map pinning system outlined in the OP:
Don't know if I understand you correctly. You would have to take a look at the files in the mod folder.
For example in MapPinPopup.lua, at the top there's a line ICON_DISTRICT_HOLY_SITE (among others), which adds the district icons to the popup.