In what directory can I find the list of terminal commands or the set of scripts that give each one its purpose/power?

Question

Is it hidden or accessible?

Also, can you create a new script and define a new command to execute system-wide by adding it to this directory and giving it the same privileges?

Answer 1

Some commands are built-in; they don't exist on disk, but are executed directly by the shell. These will be documented in the shell's man page or other documentation.

Other commands might exist anywhere on disk, but typically will be stored in a directory which appears on the search path. This is represented by the PATH environment variable, whose value is a colon-separated list of directories. Any command which exists in one of these directories can be executed by typing its name; the shell will check each directory in the path in turn for an executable by that name.