does wearing a mask mean you are breathing in carbon dioxide?

Perhaps a tiny bit more than usual but the masks aren't air tight to oxygen will get into your lungs.

Yes and basking in your own germs and any one else's that happen to Get close to you. Yummy, huh?

Not a simple surgical mask, no.

Yes and less oxygen to the brain.

Nope. Masks are permeable to the molecules of gasses involved in respiration. They stop droplets carrying virus.