Actually the article does explain this in the section "Putting it All Together: systemd".
There are different types of entities systemd is aware of.
The "service" unit and the "socket" unit are what describes the interface to a daemon. Each socket description needs a matching service description.
A daemon then connects to a socket, and systemd blocks that connection until the corresponding service is started and then hands off the socket to that daemon (there comes the patching into play).