Chatgpt is not one model! Unless you manually specify to use a particular model your question can be routed to different models depending on what it guesses would be most appropriate for your question.
Yes but then what does the grandparent mean with “unless you specify a specific model” ? Do they mean “if you select auto, it automatically decides between instant or thinking” ?
If you have the paid subscription you can choose what model your question is routed to. Current options in the UI are GPT-5.1 Instant, GPT-5.1 Thinking, GPT-5 Instant, GPT-5 thinking mini, GPT-5 thinking, GPT-4o, GPT-4.1, o3 and o4-mini. Options like deep-research will affect the reasoning level used. There is a lot that goes on behind the scenes in the chatgpt app with things like tool use or function calling coming into play as well. Ultimately what OpenAI will be trying/hoping to do is give you a satifactory result using the least amount of compute possible - this is where the autorouter is very useful for them and obstensibly for the user who would not know which one to pick. I mostly just use the API's these days as I like to be the one who decides who/what I am talking to.