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.