The wearable trackers sense your movements on a three-axis accelerometer. When you wear the tracker, the data is recorded at all times, and also it is powered up and enables the tracker to trace you when you are walking, running, and even when you are standing still.
The trackers have storage, and they store all of the data. Usually, they store the data for 30 days. It all depends on what kind of tracker measurement device you have. You can always use your smartphone tracker also, but it might not be as accurate as the professional fitness trackers.
Also, if you connect your tracker with your laptop or smartphone, all of your moves are recorded and are run through a personalized algorithm. The algorithm makes it possible for the software to detect your movements.