system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If dogs can smell some odors as accurately as one part in a trillion, how do they distinguish that one part from the other 999,999,999,999 parts?

This is more of a question of how they analyze the available data rather than how much data they gather.
gpt
You need to not think of this in terms of how *you* and other humans smell things, but how a dog smells things.  

As vision is your primary sense, let's use that for comparison.   
Stand outside and look at an enormous building, then look at a flower growing beside it- you can easily distinguish the two.   
For a dog, odors are the same way- they can coexist but remain entirely separate and distinguishable.