Valid question, as they already have a partnership with OpenAI to use ChatGPT in Siri. I personally use GPT for illustrations and Nano Banana for photo edits (Midjourney for realistic photos).
As an aside, perhaps they're using GPT/Codex for coding. Did anyone else notice the use of emojis and → in their code?
Someone who works in AI told me they think that was trained in as a "watermark", apparently the same is true with the em-dashes, to "ease people into AI" or something.