They will likely have some Apple specific features, but I doubt they'd want to make them incompatible to the point of requiring a software rewrite compared to other ARM chips. Just recompile your stuff, jump through whatever hoops you need to publish it, and that should be it.
Going by their iPhone chips: nothing special to worry about, other than page sizes which may not be an issue in the future depending on how Rosetta 2 works on their actual shipping hardware. There are some proprietary MSRs but they’re not for your use ;)
Apple licenses the ARM CPU architecture from Arm Holdings. My understanding is that Arm Holdings doesn't sell any hardware themselves.
Arm sells both ISA licenses (which is what Apple has) that allow you to make a fully custom microarchitecture, and pre-made cores (which is what e.g. AWS has, along with all the little embedded SoC manufacturers). But yeah, they don't really sell physical hardware.
Apple chips have one custom extension (AMX), but IIRC, they don't give third party developers access to this, so from a developer's perspective it's a standard ARMv8 chip.
Pure speculation on my part, but I think Apple wouldn't want people installing either other OSes on their machines or their OS on other hardware, so I would expect some lock-in hardware/firmware/software to make impossible or really difficult to build ARM hackintoshes and the other way around. Hoping to be wrong, though.
I hope so, too!
In his interview with Gruber, Federighi explicitly said that they wanted hobbyists and hackers to be able to play with the Mac hardware, so it would be possible to boot other OSen. Don’t have a link right now, but it was clear that he was saying that the platform would be hackable.