Thanks to the mod “wrp world role play mod (by agentadkiller adrien)” I learned how to write Additive Data Mods in AOMR. The Base file name must be suffixed with “_mods”, for example, if the Base file name is “proto.xml”, then the Mod file name is “proto_mods.xml” and the Root node name is <protomods>. If the Base file name is “techtree.xml”, then the Mod file name is “techtree_mods.xml” and the Root node name is <techtreemods>. I haven’t tried with other files, but I imagine it’s something similar.
I would like to ask the devs for a new article dedicated to Additive Data Mods in AOMR, like the one we already have for AOE3DE Additive Data Mods – Age of Empires Support
PS: For some reason “stringmods.txt” must be written without “_”. Maybe there are other cases like this.