There are many ways to design a computer-controlled Building Management System. It could be as simple as a cabinet of relays switched by an I/O port card, or as sophisticated as Ethernet devices addressed over the building's Local Area Network. There is no way we can tell without looking at the system or reading the installer's documentation, unless there are clues in the software configuration. Exception to the previous statement: if the system is a commercial off-the-shelf design, somebody might recognize it from the brand name and model of the software or the control cabinet.

If you don't have the documentation, get it. (Ask the building engineer or architect if you can't find a product name.) You'll need it to do your job effectively without wasting days trying to reverse-engineer the system when something goes wrong. In the meantime, try looking at the cabling coming out of the computer to see where it goes, or you could look at some of the controlled devices to see where and how the control signal comes in. That will tell you what the physical layer is like.

You may be able to look through the BMS software to see how it works also. When you get brave enough to experiment, do something relatively harmless like turning lights on and off after hours. You don't want to accidentally call in the Fire Department!