Ok, to find out more, you could first read kernel docs and then google for the following filesystems involved: "initramfs" "rootfs" "tmpfs" (ramfs is related but not used in TC).
Booting with 'syslog' and then examine /var/log/messages could give you some insight about what is happening.
Resources are freed after switch_root has been executed.
IIRC - it is a long time - when booting from initial ramdisks, they used to end up being mounted on some mountpoint on the root fs by default or at least in some cases.