They're tracked internally. By various forms of technology and automated software that communicates with the tracking devices on their person, in their vehicles, in their body cams, in their phones, in their comms systems, and sometimes even in their weapons. This metadata is commonly cited in court proceedings and is also used to track officers' whereabouts so they always know who is where and what they are doing. It also makes for easy identification of which officers are taking which actions in large-scale operations and/or undercover operations where camera footage alone doesn't tell the whole story.
The technology used by law enforcement agencies has gotten so complex that there are actually software programs whose entire utility is to organize all the data from all the other tracking systems into one cohesive and coherent integrated dataset.
This is all very good for the accuracy of charging the right people with the right crimes and ensuring that LEOs are closely monitored while performing their duties.