Flight explained
Why aircraft lower their landing gear before landing
When landing gear is extended, why it makes noise and how the system confirms it is locked.
The short answer
In cruising, the chassis is recessed to reduce air resistance and fuel consumption. Before landing, the crew produces racks at the calculated stage of the approach, when the aircraft already reduces speed.
When the chassis is released and how to check its position
The opening of the flaps and the movement of the racks change the air flow, so in the cabin you can hear the hum and mechanical sounds. After release, the locks are fixed by the chassis, and the indication in the cabin confirms the safe position.
The wheels take the mass of the plane after touching. Brakes, spoilers and reverse traction together slow down the liner; the main braking force at low speed creates wheel brakes.