I believe they are involved in some sort of pre-boot procedure to the point where a macOS install isn't just a hierarchy written to root (so "seamless updates" wouldn't be as simple as using snapshots). I could be wrong, though. /end

Tangentially, from what I understand, there are these things called Cryptexes. They are cryptographically-signed file-system hierarchies. There is the App cryptex (hot-patchable with RSR's) and the OS cyrptex. /2

Not exactly the same, but macOS does have the concept of in-place updates (no reboot necessary) called Rapid Security Responses, but they are limited to specific things and Apple hasn't really used them in a while. /1