You can paste a plain text from other applications to an article.

Select the content in another applications, and then put it onto the clipboard by pressing CTRL+C.

Click in article text where you want to paste the content and then press CTRL+V.

The text pasted from other applications is always pasted as plain. Have in mind, however, that text from other applications may contain inappropriate characters, e.g. straight quotes and apostrophes instead of the curly ones, or have all lines ending with paragraph sign, so the results may not be what you expect.

In that case, see Clean text and set quotes and Get rid of extra paragraphs.

When pasting a text from GN4, it is pasted as formatted, unless you paste it with SHIFT+CTRL+V.