Well, I would say AI integration is pretty minimal in iMessage. Apple used to call it Machine Learning, where your iPhone would try to predict your response at the top of the keyboard. They basically enhanced that feature, and changed the color to rainbow to show that it's "AI" now, but it's been like that for awhile. As for other AI changes in iMessage, you can highlight your text and have Apple Intelligence rewrite it in a professional or friendly tone. I can't say I have ever used it except for testing it the first time it came out. And of course notification summaries, which are not iMessage exclusive, is where Apple Intelligence can summarize your texts if you get a bunch together. Sometimes the summaries are helpful for a quick glance, sometimes they are not helpful and miss the point. As far as I know, ChatGPT doesn't have any access to iMessage. Your iPhone will only use ChatGPT if you ask Siri something that Siri cannot answer, at which point it will ask if you want to answer your query using ChatGPT.
