https://www.livescience.com/how-do-step-counters-work
Stephen Intille, an associate professor of computer science and health sciences at Northeastern University in Boston, told Live Science that the vast majority of step counters use accelerometers — a type of motion sensor.
The accelerometers, which are often strapped to a person's arm in the form of a watch, look for change in acceleration, said Intille. As a person moves, the acceleration of their body moves.
"If you think about that as a little curve, you are going to get little peaks and valleys every time that you step and go," Intille said. "That's your body going back and forth." He said that the algorithms inside the step counter look at those peaks and valleys to try to figure out the frequency that they occur.
"After the accelerometer sees some amount of frequency of movement that looks like walking or stepping, it starts to count each of these peaks, which end up being a step."
A different site (that in my estimation is a bit muddled including making spelling mistakes) says some use GPS and some use accelerometers, but then it has this:
There are lots of pedometer apps for cellphones that make use of built-in accelerometers. 1) This one's called Pedometer 24/7, runs on the Apple iPhone and iPod Touch, and seems to get good ratings from its users. As you can see here, it displays steps, average speed, total distance, and calories burned. You have to key in your height and weight to start, to give it a rough idea how long your steps are, and you can fine tune the sensitivity as well. 2) Google Fit is a more sophisticated excercise tracker that can figure out how much exercise you're taking for 120 different activities, including several different types of walking.
One way to check which is which is to run in place. If it counts steps running in place or doing jumping jacks or running on a treadmill, then it is not GPS. Another thing is that vehicle travel (car, bus, train) do not count as steps.
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