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File System

/bin — Essential command binaries
/boot — Static files of the boot loaded
/dev — Device files
/etc — Host-specific system configuration
/lib — Essential shared libraries and kernel modules
/media — Mount point for removeable media
/mnt — Mount point for mounting a filesystem temporarily
/opt — Add-on application software packages
/sbin — Essential system binaries
/srv — Data for services provided by this system
/tmp — Temporary files
/usr — Secondary hierarchy
/var — Variable data

etc

This is the nerve center of your system, it contains all system related configuration files in here or in its sub-directories. A "configuration file" is defined as a local file used to control the operation of a program; it must be static and cannot be an executable binary. For this reason, it's a good idea to backup this directory regularly. It will definitely save you a lot of re-configuration later if you re-install or lose your current installation. Normally, no binaries should be or are located here.

https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/etc.html