Hello Giovanni
OpenDayLight is just one of many available SDN controllers/frameworks on the market. The controller itself, which is essentially OpenDayLight software running on a server or VM, connects to the actual physical network devices using some SDN protocol such as OpenFlow. (The network devices must support this). The network administrator connects to the SDN controller via APIs, and can use a web interface, a mobile app as well as programming languages (such as python) to interact with the controller, and to automate network maintenance and modifications.
The software doesn’t create a virtual network device, but a centralized location from which the whole network can be controlled and configured as a single entity.
The introduction to SDN lesson explains this whole idea in a very clear and understandable way. Take a look at this first, which approaches SDN in general, and then take a look at the OpenDayLight lesson once again, to see an example of a particular SDN solution in action.
I hope this has been helpful!
Laz