how does laptop regulator circuit works
You're asking what the lower MOSFETs are for. This is a synchronous buck regulator, and now you know the right term, you can read about them to your heart's content. The MOSFETs take the place of the diode you see in a normal buck regulator. This allows a lower voltage drop which can be significant in a low voltage power supply. The tradeoff is complexity.

There's a hint toward the term synchronous rectification in the name of the ISL6207 driver IC, from the datasheet: "High Voltage Synchronous Rectified Buck MOSFET Driver".