815 B
After basic bootstrapping, the source for the rootfs needs to be found, via hardware coldplugging and if wanted, a device manager to extend what the kernel provides by default.
This is provided by nlplug-findfs on Alpine Linux.
Once the block is present it can be mounted, but care must be taken so that the filesystem is either autodetected or provided to the mount call.
Once sysroot is mounted, all special filesystems that have been so far mounted and optionally everything else must be moved over to new root.
switch_root claims to move /dev, /proc, /sys and /run but this is probably only true for the coreutils switch_root. Care should be taken that these mounts are moved manually ahead of time to ensure presence.
switch_root must be exec'd to overtake PID 1 and continue the init in the true rootfs.