It has three sensors that notice if it is changing speed up/down, left/right or back/forth.

A step will result in a speed increase up (foot go from still to up), followed by a sharp speed increase down (foot go from up to down), then "up" again (foot go from down to stop).

Going up a stair causes the same but different timing between the speed changes.

If you are on Android, the Physics Toolbox Suite can let you look at the exact values it is measuring.

The most basic way to measure movement is with an accelerometer. It's a little component inside your phone that has a small weight with a known mass connected to springs. When the phone moves or rotates, the weight moves, and the tension on the springs changes. The tension is either constant (you rotated your phone and are now holding it in the new position) or temporary (you moved in a direction and stopped). There are other ways this can be done, but this is the most conceptually simple.

Steps, length of step, distance moved, and heart rate can be estimated from analyzing the movement in various ways.

For example, to detect a step, your phone might see movement slightly up and forward, then down, then a jarring impact. Heart rate can be estimated based on your entered weight in an app, your speed of movement, how long you've been moving, and averages for people of your weight moving in those ways. This is a very inaccurate way to measure your heart rate, however. A better way would be by a sensor located on your wrist, arm, or chest, which is what smart watches often do.

Movement measured by an accelerometer can quickly become inaccurate, because small errors add up over time, so for movement over longer distances, phones generally use GPS (communication with a satellite positioning system) which is accurate to within about 5 meters.

If GPS isn't available, but the phone is connected to multiple cell phone towers, then it's possible to triangulate the position of the phone given the tower locations. If we know the distance and direction to the towers, and the position of the towers, then we can find the location of the phone by basically adding an offset to one of the tower locations.

There are other, more niche ways to measure positions without triangulation or GPS, but they're generally used for autonomous robotics - laser positioning with reflectors, ultra-wide-band positioning with special sensors, or visual positioning with cameras surrounding the region in which the robot will be working.

Let me know if you have any further questions.

Your phone has something called an accelerometer in it, it's a sensor that can tell the orientation of the device. It tracks your rhythm with the phone swinging left and right to tell when you take a step or climbed some stairs. Many apps also use your GPS location to give you a rough estimate if your accelerometer isn't up for it.

I was out walking with a friend the other day and he tripped and fell. His watch told him, “It seems you’ve fallen sharply.” He had to tap the screen to stop it calling the emergency services. The other friend walking with us said his wife’s watch tells her to stand up if she’s been sitting too long. “And she does it!” I’m officially a curmudgeon, grumbling about tech taking over.

Magnets.

Seriously.

So, you are supposed to calibrate the pace length, it can guess that but it’s not as accurate.

It uses accelerometers for “fine” movements like bouncing in your pocket or being flipped over (or for augmented reality, etc,) but gps for very coarse positioning. (GPS knows you’re in a building, or maybe one of a few small buildings. WiFi location knows what room you’re in.)

They use everything together to create a picture of your movements.

Check out this awesome video about how the MEMS device in your phone works with a giant 3D printed version.

Step counters use the phone accelerometers which the phone also uses to notice its own orientation. They measure longer distances with GPS. I haven't yet seen a phone that can detect your heart rate by itself. Some wristwatches/fitness bands do it using an electrical sensor (electromyograph) on your wrist, and send the info to your phone by Bluetooth.