On devices which can support Apple Intelligence it can be enabled in Settings / Apple Intelligence & Siri. Once this is turned on and a download is complete Siri will be able to call on Apple's own AI for assistance and other Apple Intelligence features wil be available, including writing tools and the option to summarise email messages and notifications. We'll concentrate on Siri here. If Siri can't handle a request locally on your device it will first send that request to Apple's AI servers. If no answer is available there, Siri can optionally send the request on to be dealt with by another AI service, called an Extension. At present, there is only one extension available, ChatGPT, one of the leading generative AI services. As well as having your requests passed on to ChatGPT automatically by Siri, it is also possible to instruct Siri to send a request to ChatGPT directly.
The settings which enable and configure access to ChatGPT are at Settings / Apple Intelligence & Siri / ChatGPT. You can turn on access to ChatGPT here and also decide if Siri needs to request your permission to send requests on to ChatGPT.
By default you will have anonymous and totally private access to ChatGPT and Apple state that OpenAI will use the information you supply with your request only to answer your request. If you create either a free or charged ChatGPT account your conversations with ChatGPT will be saved in your ChatGPT history and your interactions with ChatGPT will be governed by OpenAI's terms and conditions and privacy practices. If you want to view your ChatGPT history it may be viewed in the ChatGPT app which can be instlled free from the App Store. Your history can be deleted in the data controls section of settings within the ChatGPT app.
ChatGPT has basic services and services with "advanced capabilities" which can give answers to queries about current information. Your access to advbanced capabilities is controlled by an unspecified daily limit. If you buy a ChatGPT subscription then you will have increased access to ChatGPT's advanced capabilities through Siri and will also have additional features available only through the ChatGPT app. I have a free account and so far haven't exceeded my daily limit and I use ChatGPT quite frequently.
One final word before I give some examples. ChatGPT uses a technique known as generative AI. Basically it generates every response from scratch from its huge knowledge base, not by parroting back pre-written responses. This is great but can occasionally lead to what are politely described as hallucinations. It can make mistakes and can sometimes generate convincing but totally incorrect responses. You will get a warning about this every time Siri gives you access to ChatGPT. If it's important, check.
So what can it do?
If Siri can't handle a request for information using Apple's AI then, so long as your settings permit, your request will be passed on to ChatGPT.
If you get one of those "Siri found this on the web; check it out" responses and you don't fancy following the links you can instruct Siri again and this time instruct it to ask ChatGPT your question.
If you have a document open in the Files app you can instruct Siri to do things like ask ChatGPT to summarise this document. It works on web pages too.
If you have a photo or illustration on the screen you can instruct Siri to ask ChatGPT to describe it or answer specific questions about it.
ChatGPT may also be able to help you understand the layout of a puzzling app screen or web page.
And on it goes, limited only by your imagination.
If you want more detail about how Apple shares your information with ChatGPT check out the Apple support article about ChatGPT integration into Apple Intelligence