I am trying to understand the updating system for MacOS Catalina.
Obviously, there has to be some way for the App Store to force the root directory to be writeable. How does it do this? Does it send an ipc/rpc to like a userspace driver or middleware system?
The mechanisms used here are not documented because they have changed in the past and it’s easy to imagine them changing again in the future . Why do you care?
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
 macOS started with a completely mutable system volume, then got SIP, then got a read-only system volume. Although Apple has not made any formal announcements about this, the trajectory here is pretty clear.