Humans constantly breathe in oxygen yet the amount of oxygen in the atmosphere remain relatively constant explain why

Oxygen is produced by plants and green-blue algae and consumed by weathering and burning. And yes, also by animals breathing, but that's actually pretty marginal. Oxygen levels are far from constant. There is evidence that in the last 500 million years it fluctuated between 15% and 30% percent.

Hope this helped!

Oxygen is constantly being produced by plants.