I've seen three types of fire/overheat detection systems used on corporate to commercial sized aircraft.
Systron-Donner systems are used extensively on GE engines. I've seen them on Pratts also.
I've explained how they work
The easiest way to identify a Systron is the white circle terminal connection point.
GE CF6-80A engines use three equal resistance elements. 4.5K each - 1.5K total in parallel.
Kidde detection systems are hooked up in a series circuit. An example of one is explained
A couple images of Kidde systems.....
The connector and crimp die for the image above.
Finwal detectors are usually single probes. I've seen these used on Gulfstream aircraft for duct overheat and APU fire systems.