My source is "I know somebody who knows somebody who works in Apple's hardware design team" so take it with a grain of salt but it seems that their core is almost entirely custom, or at least customized to the point where it's effectively a different core.
And it makes sense for them to do it too, they clearly have the money and people to pull it off and it gives them a big competitive advantage. It's not for nothing that they push for ARM on the desktop as well, this way they'd have a tighter control on their ecosystem than they've had for decades (maybe ever), both soft- and hardware.
I'd be interested in being pointed to any references as to what they've actually done, not that much information is likely to be public domain.