So how would one code something similar to this? Like, which script does it impact and such?
As you have found the modding help topic, I suspect that you have found the wiki (https://strive4power.fandom.com/wiki/Modding_Guide), which explains most things about modding. Though it is slightly out dated as the recent version of Strive added some new features, which are explained in the in-game version of the modding guide. The Strive Discord(https://itch.io/t/284398/discord) is a good place to get help.
There are many approaches to modding files, though some are more likely to be compatible with other mods. Some approaches are straight forward like inventory.gd, which just replaces existing functions. Though custom files like MAD.gd can use references to adjust game data in almost any script.
The trick with using this mod is understanding what the mod was attempting to accomplish and what has changed since then. It seems that about half of the changes made by this mod were simple quality of life changes to the code, but since then the code has been upgraded causing it to be either harmful or redundant. The content added by this mod is mainly found in MAD.gd, sebastian.gd, abilities.gd, and the Images folder. Outside of that the only useful parts seem to be calls to those first 2 scripts.