I am not sure this requires any introduction !
While the ChatGPT app can be accessed through, the API portal is at platform.openai.com, and even though the website provides a free tier, the API does not, you must have prepaid credits in order to use it, the minimum is $5, those $5 are pay as you go, you use them up by sending more API calls.
GPT 4o is rumored to be 10 Trillion parameters !! for comparison, deepseek, which is an amazing LLM that came out lately is 671 Billion parameters, meaning it is 7% of the size of ChatGPT !
Models and API pricing