What do you mean source? It's a language model that learned from what people said. No source is needed, just an understanding of how llms actually work. When you ask an llm what the answer to a math question is, it doesn't run a calculation of that question. Instead of gives you back what it thinks you want to hear. Some llms have gotten additional actions like making these calculations but for the most basic implementation it's telling you want you want to hear through a series of tests that you've told it if it was right or wrong on.
So you teach it what your want to hear and it repeats it.