Hooks
The basic concept of hooking is common across all of computer science. To put it simply, it's a method of changing the behavior of an existing program by intercepting calls to functions.
In Battle Brothers we are fortunate to have effectively unlimited access to modifying the BB Classes created by the game thanks to Modern Hooks, conceptually based on Adam Milazzo's original modding script hooks. This means that any game logic handled by squirrel code can be changed without requiring any replacement of files. Additionally, by loading JS files, we are also able to modify JS objects in the same way.
If you'd like to start hooking files to make your mod, start by reading the Modern Hooks documentation.