Creating an email for domain alias in G-suite Legacy

I have succesfully added a new Domain alias, but when I try to create a new email at the @alias-domain.com I am not allowed.

I then noticed that there are greyed out emails @alias-domain.com with a short description saying

"This domain alias is automatically generated."

So that explained the things...

If you add a "domain alias" you are not allowed to create a user for it as for a second domain.

It just copies the already existing email addresses.... or something like that