Use :verbose map to show where key mapping(s) are defined (see :help map-verbose). There are other variants as well that can be used with or without verbose:

:nmap for normal mode mappings :vmap for visual mode mappings :imap for insert mode mappings

To output the shortcuts, with where they were defined, to a text file do:

:redir! > vim_keys.txt
:silent verbose map
:redir END