Mostly technical fixing/feature improvement, though there are actually quite a few mods, of various kinds for Odyssey of Gianna, their ability to run together, done thanks to a mod loader, that in itself started as a "mod", by another community member, but got added to the base game in a later version, as otherwise normally, yeah more than a single mod at least is basically not possible on an RPGM game.
The game even got a wiki, with a section for mod pages, so the 2 mods I made (with the latter depending on the former) are https://gianna.miraheze.org/wiki/Modding:GUM_Core and https://gianna.miraheze.org/wiki/Modding:GUM_ReEquip if you want to see more specific information
Can ofc find links to the game itself, if you go back to the main page on that wiki too. Since it is a paid for fetish game($10 for base game + $5 for the DLC), you wont be able to find the store page by searching on itch, thanks to that whole payment processor situation.