Pressing C-x C-e opens the current commandline in your default editor.

As in what's in your $EDITOR variable. If you haven't touched it, it's most likely Nano or some minimal vim

I find that if your command is complex enough that editing it on the terminal becomes annoying, then there's a very high chance you want it in a file anyways, just to document what you did and to allow easily re-running it.

Having said that, you can also have your shell open the command in your editor of choice: https://www.stefanjudis.com/today-i-learned/edit-long-shell-commands-in-your-usdeditor/

Couldn't you write in the text editor then copy/paste into terminal without saving? (Who needs documentation anyway)

I write in Notepad++ then copy paste to WSL.