They have an architecture license, so can do quite a bit beyond simply "tweaking" it. They have some degree, presumably a significant degree, of "blank sheet" implementation.

At this point, probably the main benefit of the ARM license is just the patents.

I have read that the architectural license comes with access to ARM engineers expertise, internal build tooling, validation and testing systems etc. So whatever processor design Apple may be doing, they are definitely doing it with the guard rails on.

Those few architectural licenses are secret, but surely, as you say, allows Apple to discuss all sorts of things with ARM’s engineers. I have long wondered what constraints it might carry. They have added an instruction or two, but I doubt there is any real reason for them to ever want to make significant instruction set changes.

Apple has a longstanding relationship with ARM back to the latter’s founding. The sale of its large chunk of ownership in ARM helped Apple survive in its late-90s reboot. So it’s quite possible that their license is quite different from those of the few other hardware licensees.

Also, Apple plays the long game. I’m sure they also have a lot of RISC-V experience by now. Not just weirdo in-house research products; I’d be shocked if there weren’t tiny RISC-V cores doing small housekeeping tasks inside some of their huge chips, just for the experience. Maybe even a few inside an Apple pencil or maybe an ipad screen digitizer since the computation isn’t really user visible.

As does ARM: if RISC-V ever becomes airborne (it’s still speeding down the runway) I suspect ARM will haul out a stack of patents and start demanding payment. Apple’s license surely covers them here.