It’s all well and good to build a login page, but you need some way to tell WP to actually use your custom page.
Luckily, there is a way to do that, with a filter (of course):
add_action('init','redirect_login_page');
function redirect_login_page() {
// your login page code
}
Put your code (or call a template) inside the function. And place the above code in your functions.php (in your Child Theme, hopefully, since you don’t want a theme update to overwrite your custom code on a theme update).
More details are available here https://premium.wpmudev.org/blog/customize-login-page/ and in the Codex https://codex.wordpress.org/Function_Reference/wp_login_form, plus lots of other resources about customizing the login page on the googles.