Configuring QGIS for external editor

QGIS uses editor set in the operating system. If you use Linux, you can set the VISUAL and EDITOR environment variables, for example to use vim as an external editor:

export VISUAL=vim
export EDITOR="$VISUAL"

If you would like to set it permanent add these two lines above to the .bashrc file in your home directory. Replace vim with your favorite text editor (e.g. nano, pico, gedit, etc.).

On Windows you can set default application depending on the file extension. Here is an example: https://www.howto-connect.com/make-notepad-as-default-text-editor-windows-10/