The old way of modding

OpenIV set the standard of modding by providing a way to replace files without the R* launcher removing the changes.

They achieved this by redirecting the game's file reader functions to a custom mods folder as you can see here: https://github.com/martonp96/ClosedIV/blob/master/src/hooks/FileReadHooks.cpp

Basically the game is looking for a file, then the redirected function checks if it exists in the mods folder, if yes use it, if no then keep looking for the original file.

Logging this process looks somewhat like this when the game is started: https://pastebin.com/2rGdYnar

The drawback of this, is that you have to copy and edit entire RPFs, even if you want to change a single letter in any file.