20 min Video that explains accelerometers and gyroscopes in MEMS chips. MEMS means Microelectromechanical systems. So there actually are mechanical systems, that work in your phone, etched into silicon chips. The video uses 3d printed models to explain how the accelerometers work, the gyros are also explained, but a little shorted, because they are very complex and therefore hard to explain.

It used be a droplet of mercury inside a sphere with electrical contacts. I doubt they'd use that method today.