/usr directory
/usr directory in Linux doesn't stand for user but for Unix System Resources.
symlinks to other folders
/bin, /lib and /sbin are just symlinks to /usr.
So /usr is the single directory that contains all libraries and binaries that comes with the operating system.
Where it's used
The directory is used by all userspace applications and programs.
- It's available for the systemd to use everything from here during the boot.
- Also all installations from package managers such as apt, yum, dnf, etc goes here.
Merged directories
Earlier, /bin, /lib and /sbin were separate directories, but they have been merged into /usr.
Currently they're just symlinks to /usr.