It's not reasoning, it's predicting likely text, which it can do because the bits of input you provide overlap with enough bits in its training corpus, because

It's not reasoning, it's predicting likely text, which it can do because the bits of input you provide overlap with enough bits in its training corpus, because most programming tasks are repetitions of similar patterns. The outcome approximates reasoning in some cases but the process is completely different. It is not building up from principles, it is sifting down from examples.