Quick answer: A pet GPS tracker has a built-in SIM card that receives GPS satellite signals and sends your pet's location to your phone via cellular network — the same way a car GPS works, in a small wearable.
The device receives GPS satellite signals to determine its location, then sends that data through a mobile network to your phone app. This is why most trackers require a monthly subscription — they use cellular data. Update frequency ranges from every 2 seconds to every 10 minutes depending on the mode.
Devices like Apple AirTag don't actively broadcast location. They're detected by nearby Apple devices that anonymously relay the position to you. No subscription needed, but range is limited without other users' devices nearby. Best for finding a lost pet in populated areas after the fact.
More frequent location updates means better accuracy but shorter battery life. Most trackers offer a power-saving mode (update every few minutes) and a real-time mode (every few seconds). Real-time mode can drain a battery in under 24 hours.
Cellular GPS is the most reliable option for real-time tracking. Bluetooth community trackers work well as a backup or in high-density urban areas where coverage is dense.