Wordpress Multisite: how to add a site to an existing one?

Question

WordPress

Good afternoon.
There is a site mysite.ru. I wanted to add subdomains to it, which will be copies of the main site, only with different city names.
A subdomain with the city krasnodar.mysite.ru has been registered.
It is necessary, using one admin panel of the main site mysite.ru, to connect domains of other cities somehow. Those. without getting into any FTP, without uploading anything to any real subdomains, stupidly adding a subdomain in the mysite.ru site admin panel, having a working subdomain-clone of the main site with one common admin panel and theme, automatically replacing only the city name on the subdomain (possibly in the future some other parameters such as address, phone number, etc.).

I googled, I use Wordpress Multisite (i.e. an option in wp-admin, I didn’t reinstall the site in any way). Added a subdomain there. Perhaps you need to specify which theme to use? But the existing theme on the site is not displayed here:

You can’t add it either, because. it writes that such a topic already exists. In the settings of the subdomain site, I specified the name of the topic here


The site itself krasnodar.mysite.ru is empty (blank page).
Is it possible under such conditions to do what is required?

I'm not savvy in server matters, I decided to try not to refuse, if not too difficult.
If it's only about the checkboxes in the admin panel, then it's fine. If it's already about server settings, then I'd better refuse this business, because. will not be tough.