Apple’s ARM chips contain an awful lot of different components, some of which are highly customised. The neural engine stuff they use for image processing for example. The CPU cores are custom tuned, but I believe they are still 90% based on ARM CPU core designs. For example the Secure Enclave is actually an ARM developed CPU extension. Apple focuses their efforts vary specifically on areas that give them features that grant a competitive advantage. They don’t reimplement stuff for the sake of it. One of their big performance wins over other ARM licensees isn’t even really in the CPU, it’s the very well designed and generous cache memory systems.

Having said that, looking to the future I think Apple is clearly willing to pour more resources into their CPU programme than any of their competitors. This means the risk to them is probably a lot less than to say Qualcomm or Samsung as they have a much better capability to go their own way. As time goes on their designs will probably diverge more and more from main line ARM anyway.