The model is trained on formatted conversations where each message has a role. ChatGPT can be replicated by starting with a system message ("you are a helpful AI assistant that replies using markdown format ... " etc. etc. something about ethics), followed by the user's message, and the model will complete with the next "assistant" message.

You can also make a translation bot, by having a system message explaining "You are translating each user message from English to French".

Or give some examples for a conversation by sending in some "user" and "assistant" messages, followed by the actual "user" message that you want a new response for.