The Filesystem Hierarchy Standard[0][1] is used by most Linux distributions as the authoritative source and description of the various system directories. As other commentators have already noted, the hierarchy on UNIX system has always been a bit of a mess, the FHS is an attempt to clean up while maintaining backwards compatibility.

[0] https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html [1] https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard#...