Apple Intelligence features were added starting with iOS 26, but in its current state, they are not very usable.
With the following settings, you can make ChatGPT the default conversation target and also save your conversations.
At the moment, Apple Intelligence has a bug where conversation scrolling does not work properly, so if you do not save the conversation, you may not be able to view it later.
Note that only the most recent conversation is saved. I wanted to save everything, but I could not find a good way to do that.