Unique wordpress multisite requirement

You would do this by editing the site in the network admin, and setting the sites URL to subsite.main.domain, WP doesn’t force you to have everything as a direct subdomain, you could even put completelydifferentdomain.com as the URL and as long as your server is set up to route requests to WP from that domain WP will handle it just fine

Subdomains are just the default, a suggestion. In the old days you might have needed a plugin such as domain mapping, but most of that functionality was put in core along time ago, you would only need it if you needed alias’ where a single specific site is served on multiple domains at the same time.