how to email username and password to admin of new registration

You can use an action to do that, but you can’t get the password because it’s encrypted, unless you catch the password when the registration form is submitted but that’s not a good practice.

Why do you need to get the passwords on admin email?
You can always reset password if the user forgot the password.

To send user information when a new user is registered:

add_action( 'user_register', 'send_new_user_info_to_email', 10, 1 );

function send_new_user_info_to_email( $user_id ) {

    $user_info = get_userdata( $user_id );

    $to = get_bloginfo('admin_email');
    $subject="New User";
    $body = 'User email:' . $user_info->user_email . '<br>User pass (encrypted):' . $user_info->user_pass ;
    $headers = array('Content-Type: text/html; charset=UTF-8');

    wp_mail( $to, $subject, $body, $headers );

}