When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich.
Chemistry and physics are branches of science that both study matter. The difference between the two lies in their scope and approach. Chemists and physicists are trained differently, and they have different professional roles, even when working in a team.
Chemistry is also a physical science, and could be seen as a bridge between physical and life sciences. Chemical reactions happen in lifeforms like humans constantly, as studied in biochemistry, so understanding chemistry naturally leads to a better understanding of living organisms.