There are many types of fire alarm system loops. The loops are meant to carry signals from the devices to the panel, or the loops are meant to carry signals from the panel to the devices. With many fire alarm systems, though, the loop is meant to carry signals both directions: from the devices to the panel and from the panel to the devices.
Often, the detectors and pull stations are connected to one loop and the sounders are connected to another loop, but how the devices are connected is determined by the manufacturer.
What kind of loop it is and how to connect to the loop will be different from one manufacturer and model of fire alarm system to the next. The way the equipment is connected will be determined by how the manufacturer has designed the panel, how the manufacturer has designed the loop, and how the manufacturer has designed the devices that connect to the loop.
With the fire alarm panel and with each device, the manufacturer has included an installation sheet or installation manual. These sheets and manuals show how the system is to be installed. This installation information material even has diagrams showing how to wire in the devices and the panel to the loop.
Thoroughly read the installation sheets and the installation manual. Not only does this material have wiring diagrams, but this material has explanations and specifications on each piece of equipment and device used in the fire alarm system.
If you are still having problems understanding the system, you can send me the make and model of the fire alarm panel, as well as the devices. When I know more, I can explain what's going on further.