Thanks for al your replies, but I still don’t get it.
This thing can certainly not be changed in Customizer, and changing the PHP-file, I would not know how to write PHP.
I do write html-code and find ways how to use which code in Chrome Dev Tools. Whatever I change I do it in the Child-theme I made, using Dashboard – Edit CSS.
When I look into Chrome Dev Tools to the site of illutt I can see what they have written in the footer id, but I’m not sure if that’s the way to do it. Writing html in a php-file?
If I look into the no-content-footer.php file I still don’t see how to insert text there.
Anyway, as I said before, I was just curious how to insert your own text in the credit line. But because I am looking for a clean and graphic looking design, I’m happy with making the whole credit line disappear, which I have done already.