How to edit wp-login.php if I am on //localhost/wordpress?

Hi everyone,

I am a beginner to WP, have experience with C language/ objective C. Currently I would like to custsomize my login form (https://codex.wordpress.org/Customizing_the_Login_Form). However I couldn’t find my wp-login.php files. Can anyone please advise?

I actually want to create a button, which will prompt a log-in pop-up if user isn’t signed in, or proceed to next page if it has been logged in. However I have no luck in searching any useful resources, or I am too stupid to understand what the gurus are trying to help me with.

thank you!