Make main page wp-signup.php?

Question

WordPress

How to set wp-signup.php home page in wp?

Answer 1

You have a noticeably non-standard task, so I'll start my answer with "try it."
Try wp-config.phpadding the following lines:

define( 'WP_HOME', 'https://VashSite.ru/wp-signup.php' );
define( 'WP_SITEURL', 'https://VashSite.ru/wp-signup.php' );

It is possible that only one of them - here it will be necessary to experiment.
You may also need a line

define( 'NOBLOGREDIRECT', 'https://VashSite.ru/wp-signup.php' );

or it is the same, but empty, or maybe this line, on the contrary, will need to be deleted or commented out. UPD: In the comments under this answer, I added an assumption about
define( 'NOBLOGREDIRECT', '' );
'NOBLOGREDIRECT'

Answer 2

You can just redirect unauthorized users

add_action( 'template_redirect', 'redirect_non_logged_users' );
function redirect_non_logged_users() {

  if ( is_home() && !is_user_logged_in() ) {
    wp_redirect( wp_login_url(), 301 );
  }

}