One of the attributes of a process is its controlling terminal. Child processes created with fork inherit the controlling terminal from their parent process. In this way, all the processes in a session inherit the controlling terminal from the session leader. A session leader that has control of a terminal is called the controlling process of that terminal.
You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.
An individual process disconnects from its controlling terminal when it calls setsid to become the leader of a new session. See Process Group Functions.