This kind of mod is always a DLL of some kind, and typically, they’ll have you install the DLL to a location that the script extender will load DLLs from automatically (but sometimes they instead use the same name as a Windows DLL and go in the same directory as the game’s executable, as when the game tries to load the Windows DLL, it’ll try ones in its own directory before System32 and similar folders, then as long as the mod DLL in turn loads the real DLL, everything will still work). When the DLL’s loading, it’ll either overwrite bits of memory corresponding to functions with its own code, or if it needs to replace the whole function, will swap out the first few instructions with instructions to jump to a mod function instead.