They really don't. Even per-project it can be a shitshow. For instance, while I use LLMs to write some typescript, when I'm writing code for Unreal Engine in C++ & Blueprint I pretty much do it by hand because it absolutely can't do blueprint in any meaningful sense. Even the most basic requests result in an absolute mess. (I imagine it's also very hard to train on since its stored as binary data). Even the C++ code it generates is pretty rough -- regular linker errors etc.