Subdomain redirect to website.com/wp-signup.php?new=example after multisite

I have a question and I hope that someone can help me!
I have a Website powered by WordPress with some articles and I wanted to make a Network, so a multisite installation. I followed the steps that are present on the WordPress Codex, inserting first the function to enable multisite in wp-config.php and then I went in my wp-admin to and followed the instruction to complete the procedure. This is my code added in wp-config.php:

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', true);
define('DOMAIN_CURRENT_SITE', 'website.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

While this is the code in my .htaccess:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

When I did this, then I realized that even creating a new site in the network it couldn’t be loaded: in fact, when I was going to the site test.website.com, the server didn’t responded, giving a Chrome error that now I don’t remember: it was something to do with DNS. So I’ve searched online and I read that I needed to create a WildCard. As how I understood, I needed to create a new DNS record of A type and redirect it to the IP of my website, with the * as subdomain. I did this, but I created a subdomain with cPanel too: I named it *.website.com and its folder was the root of my WebSite, then I did a redirect to my website IP for this subdomain.

For some hours anything happened: this is I guess because I had to wait for the DNS change, and in fact, this morning, I saw that test.website.com was loading!

But there’s a strange thing: if I type a non existing subdomain like wp.website.com, it redirect to my Principal WebSite (the domain in itself) with this on the link:
/wp-signup.php?new=wp
The wp after the = is because the subdomain is wp in itself.

In the page there appears like a registration module: what I mean is that if I’m not logged in my website, a page like this (that could be asd.website.com or java.website.com and redirects to website.com/wp-signup.php=?new=ads), shows up a registration requiement and if I proceed, it seems like if a new user could register a new subdomain or something like that.
Because appears a Username box and a password box, then I can click on “Next” in the bottom of the page. It feels like a registration form for my network… but it’s right that this appears when an user writes an not existent subdomain of my website?

Anyway in the very bottom of the page, there comes out (I translate from Italian, so maybe isn’t the real english WordPress text): The Website you were looking for, hello.website.com doesn’t exists.

Oh, I forgot to say: this page comes with the layout of my WordPress theme, but it’s sovrasting it: infact buttons and text box aren’t of my WP theme, but of WordPress itself.

That’s all! What I would know is: it’s a normal thing or I messed up with something? And if it’s normal, how could I make this not to happen? Thank you anyone will help me!