A gyro measures the angular velocity of a drone around 3 axes: yaw, roll and pitch, but what physical change does it make to the drone after measuring this value to give it stable flight? As it is a gyro chip, and not an actual gyroscope, would it still precess? does it change motor speed etc.? Thanks in advance.
The IMU (Inertial Measurement Unit) is a component within the aircraft which generates a signal that is proportional to the accelerations experienced on all 9 DOF (degrees of freedom). Modern IMUs are extremely sensitive to acceleration and can be calibrated to measure minute angular changes, very rapidly.
The IMU signal can be analog or digital and it is processed by a flight controller which is programmed to manage the flight behaviour of the aircraft. It does this by adjusting the control surfaces (ailerons, rudder,...) via a servo, or the propeller speed/pitch in the case of a rotor wing via an ESC or electronic speed controller.
Small "drones" (correctly called UAV or UAS) which use 3 or more horizontal propellers but no wing, only use the relative speed of the motors to manoeuvre the aircraft.
Changing the motor speed affects the propeller thrust and so each pair or motors can be adjusted to create differential torque to rotate the aircraft and unbalanced lift to change the aircraft attitude.
If a hovering aircraft is tilted, it will move sideways and therefore the IMU can sense the angle which is then used by the FC to adjust the motors to maintain that angle until the pilot moves the controls.
The IMU simply generates signals - the flight controller processes them - the ESC is the throttle that controls the motor.