I’m primarily talking about stuff like step counters, but also health apps measuring your heart rate, how many stairs you take, the length of your steps, and so on. I’m honest when you tell me to explain it to me like I’m five years old, this one boggles my mind.
There’s another trick - checking against street maps. When you walk/drive/ride around a corner the phone can use it’s inaccurate GPS reading and update it according to where the corner is. Of course it doesn’t use ONE corner. to fully update your position, it just nudges the position a bit closer.
In case you are navigating a lot of street corners, after a time an inaccurate GPS position will be corrected.
There’s even more input to the algorithm (location of known WIFI Hotspots for example etc) and they are all combined with a “Kalman filter”.
https://en.m.wikipedia.org/wiki/Kalman_filter