How does the CO2 I breathe out becomes oxygen?

Explanation:

When you breathe out air, plants take that in and use it for photosynthesis, or the production of food for plants. When they complete photosynthesis, they give out the waste product oxygen, and the cycle goes again.

Hope I helped:)

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong