the base LLM is token predictor - given some stream of tokens, guess the next token, and the next, etc. But ChatGPT is trained on top of the LLM with a whole

the base LLM is token predictor - given some stream of tokens, guess the next token, and the next, etc. But ChatGPT is trained on top of the LLM with a whole bunch of example questions+answers, and also most likely they are augmenting your prompt with additional prefix/suffix to nudge the model to output a chat-like answer/response to your input... that's my guess and you can read more in the papers on how they trained it.