/usr/share/zsh/functions/Zle/edit-command-line is in zsh-common 5.3.1-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | # Edit the command line using your usual editor.
# Binding this to '!' in the vi command mode map,
#   autoload -Uz edit-command-line
#   zle -N edit-command-line
#   bindkey -M vicmd '!' edit-command-line
# will give ksh-like behaviour for that key,
# except that it will handle multi-line buffers properly.
() {
  exec </dev/tty
  # Compute the cursor's position in bytes, not characters.
  setopt localoptions nomultibyte noksharrays
  (( $+zle_bracketed_paste )) && print -r -n - $zle_bracketed_paste[2]
  # Open the editor, placing the cursor at the right place if we know how.
  local editor=${${VISUAL:-${EDITOR:-vi}}}
  case $editor in 
    (*vim*)
      integer byteoffset=$(( $#PREBUFFER + $#LBUFFER + 1 ))
      ${=editor} -c "normal! ${byteoffset}go" -- $1;;
    (*emacs*)
      local lines=( ${(f):-"$PREBUFFER$LBUFFER"} )
      ${=editor} +${#lines}:$((${#lines[-1]} + 1)) $1;;
    (*) ${=editor} $1;;
  esac
  (( $+zle_bracketed_paste )) && print -r -n - $zle_bracketed_paste[1]
  # Replace the buffer with the editor output.
  print -Rz - "$(<$1)" 
} =(<<<"$PREBUFFER$BUFFER")
zle send-break		# Force reload from the buffer stack
 |