How to edit login form

I can’t figure out how to edit the form on my redirected login page. I’ve set up the new url for the login page which works fine, but I can’t find how to edit the form on that page that users use to login.
I have edited it in the past and customised it with my own logo, but now I can’t find it as there is no actual page in the pages section for the new login (redirected) url page.
Its been a while since I set the site up and so apologies if this is really obvious and I’ve just forgotten!