How are subdomains setup by popular websites


I would like to know how popular websites setup their subdomains.

For instance,

What is the best way to set this up with wordpress multisite?

I know that if I add a new site under my super admin account I can create these subdomains but then it also creates a whole new admin dashboard for each subdomain.

It just seems like it would be more difficult to manage this way since you have to login to each admin area. I would like to set it up so that I don't have to do this.

How can this be done and what is the best way to set it up so that it is easy to manage?

Let me know, thank you!