The principle of dividing one site into different "sites"?

Question

System administration

What is the principle behind the operation of online services (for example, ucoz), on the basis of which “sites” are created, accessible (under certain conditions) through a domain attached by the user?
As I understand it, a “child” is available on the service at a certain address, and the web server is configured in such a way that the attached domain is an alias to this address?
For example, server/ is a service, server/~user/ == userdomain/ ?

Answer 1

A user is created in the system, a database is allocated for the user, a virtual domain, ftp access, access to the mail server.
Actually, the user's home folder is the main repository of his scripts, logs and other client information.
In general, it looks like this.

Domain name: test.ru
Username: test
User password(FTP,): test
Database name: test_db
User password (MySQL): test
User quota(Mb): 50

Answer 2

You can start with this - debian.pro/558

Answer 3

the wildcard is simply hung up in dns and cms itself is able to determine which domain was addressed to.
The banal scheme for an example for domains with www and without works exactly the same