For a couple of years now, macOS has an RO System volume image, and then mounts an RW data volume on top of that, similar to how overlayfs works on Linux. That system volume isn't modifiable. If it is, then the system won't boot. So I'd say it's a little bit immutable.

https://eclecticlight.co/2021/10/29/how-macos-is-more-reliab...