Hi there! 🙂
Yes, if you make a direct edit to the Core files, the changes will be overwritten when an update happens because updates are updates to the Core files of WordPress.
Have you looked at this page here: https://codex.wordpress.org/Customizing_the_Login_Form
on ways to make edits and customize the login page using plugins or functions and such?
This might help you to make the changes you want while still keeping the Core update-ready.
That customization is too less? I guess… Because I want to make like only “Login” and “Register” text which when user press login form pops up. But I can’t like get those texts from nowhere I guess unless I edit the file myself. Or is there an actual way to do that?
Also would users have same login as me? “http://www.armandgilbert.com/wp-content/uploads/2012/11/tutorial-2.jpg” sorry if that’s a stupid question but I don’t know much about WP.