Unfortunately OpenAI has a pretty big "dollars and hours spent on GPUs" moat right now. I imagine Apple is already hard at work building their own models, but until then they will leverage 3rd parties
Apple has 10x more on reserve cash than the entirety of OpenAI when they trained GPT-4. I don't think OpenAI could have possibly spent $5B or more for training first version of GPT-4(which was trained before GPT-3.5 gained traction), which is a pocket change for Apple for such a core feature.