The first site you will encounter is known as Default-First-Site-Name. This site is created by the dcpromo process and includes the first domain controller for the domain and forest. If you never create any other sites or subnets, all subsequent DCs will be located in the default site container.
If you have precreated your sites and associated the appropriate subnets with those sites, each DC you install after that time will be placed in the appropriate site container. This assumes that each DC is configured with the appropriate IP address for the subnet.
DCs that are already installed must be moved manually if a site is created and associated with an existing subnet.
The process of creating a site is simple. As mentioned ...