Plant respiration produces carbon dioxide, so why are plants regarded as decreasing its atmospheric concentration?
There are two factors here.  First is that you have to look at the plant's net production over time.  Say for instance (I'm just making up numbers here) that during the day the plant takes in 88 g of CO2 and uses photosynthesis to convert it to 64 g of O2 that's released into the air, and 24 g of C that is incorporated into the plant.  Then during the night it uses 12 g of the stored C for respiration (combining it with 44 g of O2 from the air), leaving a net daily production of 12 g C that goes into plant growth, and 44 g of O2 in the air.

Second problem is perhaps better classed as either linguistic confusion (to be polite) or ignorance.  CO2 is not "pollution": it is a normal part - more accurately, a fundamental part - of the carbon cycle https://earthobservatory.nasa.gov/features/CarbonCycle on which almost all life depends.  The problem is not plants (or animals) releasing CO2: it's humans digging up lots of fossil carbon and burning it, thus increasing atmospheric CO2 concentrations beyond the levels to which the ecosystem has adapted.

Now that you've changed the question, it's even more important to read the basic description of the carbon cycle at the link.  (Honestly, wasn't this covered in your high school biology class?)  Again, the bottom line is that plants produce a net surplus of O2, which animals consume along with the plants, so that the cycle remains balanced.