Sorry, but I have to say I was a tad disappointed with number 4 - the explanation of the linux file system was a bit woffly and not particular clear. Much more could have been made of why there's apparent duplication, e.g. /sbin and /usr/sbin, /lib and /usr/lib
May I clarify? Historically, when disks were much smaller, the root file system wouldn't have been very big, and /usr and /opt and /usr/local/ might have been separate disks and thus not mounted when the system boots. Thus, to boot, when the only thing available is the main disk (or disk partition) you need to make all the Special/System binaries (hence /sbin) available - often these are statically linked and do not need /lib and definitely cannot use /usr/lib which can't be guaranteed to be available.
/lib also needs to be on the root disk because kernel modules live in /lib/modules, if they were in /usr/lib/modules then the system wouldn't be able to find device driver modules to load into the kernel.
HTH
Paul