You can find many modding websites and communities online, which offer tutorials and free tools to carry out mods on various game engines. So you might be better off joining one of those communities.
Probably the source code of many of these free modding tools is hosted on GitHub, but what you need most is being part of a modding community, to share their knowledge, learn and ask questions.
Modding is an art, and requires knowledge of how the various game engines organize their data. So, the first step in modding any video game is to work out:
- Whether modding it is approved by its developers.
- If there are any official modding guides and tools from the game devs or their fans (there often are).
- Discover which game engine it’s being used, and which tools are available to mod its games.