But Apple has the luxury of controlling both the server and the client. Why isn’t it just a matter of using HTTP range requests?

https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requ...

And if they need to get more clever than that, why not have a BitTorrent-like map of chunk hashes. So you download the update, check the hash of the whole thing and if it fails the hash check, download the chunk hashes which will be say a 256 bit hash for every individual 32 MB chunk of the file. And for any chunk that fails the hash check, use a HTTP range request to download it again.