That's interesting, I don't have much contact with Apple's ecosystem.

Sounds similar to a setup that Linux allows, with the root filesystem on btrfs, making a snapshot of it and updating, then live switching kernels. But there is no firmware support to make the switch, so it relies on root having full access to everything.

The hypervisors approach seem like what Windows is doing, where Windows itself gets booted in a Hyper-X VM, allowing WSL2 and every other VM to run at "native" speed (since "native" itself is a VM), and in theory should allow booting a parallel updated Windows, then just switching VMs.

On Linux there is also a feature for live migrating VMs, which allows software to keep running while they're being migrated with just a minimum pause, so they could use something like that.