[thatcks]

For switching to the real root filesystem, you want to read about the pivot_root system call, for example http://man7.org/linux/man-pages/man2/pivot_root.2.html (the usage examples in the pivot_root(8) manpage will help).

When the initramfs PID 1 is something simple like a shell script, I believe it just exec's the real root filesystem /sbin/init after the pivot_root and that init starts from scratch. For Linuxes where systemd is the initramfs init, there's a magic internal protocol to let a running systemd re-exec a new systemd binary while preserving all of its runtime state; this is used, for example, during systemd package upgrades and can be done deliberately with 'systemctl daemon-reexec'.

(I'm the author of the original entry.)