I know this has been cover 1.34 million times but im just a little confused

i coped the register.php single page and controller from the concrete folder, I edited the site_theme_paths.php and the registration page now has my theme applied to it, but how do i edit the actual content of the page.

I assumed I would edit single_pages/register.php but that doesnt work i still need to edit concrete/singe_pages/register.php

i thought the point of the steps above were to be able to edit it without the upgrades overriding the page?

Thanks in advance