Also: You boot from the 'system' volume, and the operating system is stored on the 'boot' volume. (-:

* https://jdebp.uk/FGA/boot-and-system-volumes.html

'Windows subsystem for' actually has a similar sort of logic behind it.

* https://news.ycombinator.com/item?id=11417059

Although Microsoft then spoiled that explanation a couple of years later by switching to a virtual machine system instead of the NT kernel emulating the not-present Linux kernel.