The main source of information for the dog is the smell. His brain is 10 times smaller than that of a human being, his sense of smell is 40 times larger than the olfactory part of our brain, and his ability to identify smells is 1000-10000 times larger.
It's interesting how the side slots on the nose of the dog are designed. When people inhale and exhale, air enters and exits the same way. Any odors that get into our nostrils are replaced by exhaled air. But when the dog exhales, the exhaled air exhales through these lateral incisions and leaves to the sides, so the odor goes straight into the dog's nostrils and is not diluted by the newly exhaled air.
Side cuts also help the dog to keep the odor particles in his nostrils. Exhaled air comes out of the side cuts, creating a twisted, turbulent airflow that allows interesting odors to reach the center of each nostril.