You may want to add a little note about several distros merging or soft linking /usr/bin and /bin together. The distinction used to be important when / was half a megabyte and the /usr programs were loaded off 2.5MB magnetic tape, but most Unixlikes don’t really need the split these days. The only limitations we have now are hardcoded paths, old packages, and people with Opinions.

The entire process is full of messy open source politics, but Debian has merged many of the /x directories into /usr/x (though there’s still a lot of bickering internally because dpkg can’t deal with symlinks like that and they don’t want to be forced to deal with it), Fedora has finished the merge years ago, Gentoo has a tool that will do the merge if you wish, and I believe openSUSE has a merged /usr as well these days.

There are reasons to keep /usr split from the other directories (say, mounting /usr over the network but keeping the important system binaries locally) but they’re not relevant for most users. If you install Linux today, there’s a good chance many of the directories in / are actually symlinks to their /usr equivalent.