I really wanna know how dogs decide where to piss

They prefer to (re-)mark their territory where another dog has pissed already. This creates a cumulative effect along paths that see a lot of dog walking. Plus some instinct to use places that stand out somehow (big tree trunk, cornerstone etc.).

I’m just fascinated by the concept of nonhuman sensory perception.

Highly heightened sense of smell, incl. smell memory.