What is the hook in WP to wrap the event in it immediately upon registration?

Question

WordPress

Tell me a hook in WP in order to immediately wrap an event in it upon registration? The bottom line is that the check will be on whether the user has logged into his personal account and whether he is subscribed to the mailing list, but after all, these 2 events can fire many times, but it is necessary during registration, that is, as soon as you register, there will be a check

Answer 1

There is a ready-made Wordpress hook that fires as a new user has been registered on the site. More details here https://wp-kama.ru/hook/user_register
You can use something like this:

add_action( 'user_register', 'registration_save', 10, 1 );
 
function registration_save( $user_id ) {
 
    if ( isset( $_POST['first_name'] ) )
        update_user_meta($user_id, 'first_name', $_POST['first_name']);
 
}