so I don't need to umount them manually to unplug after halt? Right?
Part of the shutdown process is to unmount all mounted filesystems.
If you disabled your "splash screen", you'd see the kernel messages going past as your machine is shutting down [ You'd also get to see the startup messages, these can be informative and useful ]. Somewhere near the end of the shutdown process comes "Unmounting remaining filesystems", very shortly after that, it halts.
Once the machine has powered off, it is safe to remove any external drives.
If you want to remove a mounted drive without powering down your machine, you must unmount it manually (ckicky-ckicky in the GUI, or the umount
command in a terminal).