But aren't they able to do incremental builds and separated x64/arm64?
They know which OS version is requesting an update, at least the version number part.
They know which OS version is requesting an update, at least the version number part.
During the PowerPC to Intel transition, they did stuff like that; perhaps at their current scale, there's reasons why they don't.
Supporting both architectures enables a macOS install to boot an Intel Mac or an Apple Silicon Mac, which is useful in a dual-architecture environment.
It's easy to check for dual architecture support; just use the file command:
$ file /bin/ls
/bin/ls: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64e:Mach-O 64-bit executable arm64e]
/bin/ls (for architecture x86_64): Mach-O 64-bit executable x86_64
/bin/ls (for architecture arm64e): Mach-O 64-bit executable arm64e