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.
Basic Mathematics are the foundation, Physics would be tier 1, Chemistry Tier 2, and Biology tier 3. So to directly answer your question I'd say chemistry is neither just biological or just physical; it lies between the two and catches the edges of both. cheers, me.