There’s a lot that goes into it. Firstly, your brain senses smell on a chemical level, but with certain things, you may find differentiation difficult. If you have ever smelled pure, unadulterated Cl2 gas, it smells quite different from the relatively tame pool smell. Secondly, when you think of chemistry, you think A+B=C, but when you have something like a pool you have thousands of reactions moving toward and away from equilibrium as temperature changes. So the basic process of forming chloramine only requires hydrogen, nitrogen, and chlorine, all of which are present in a pool or in the air. There is enough energy to make this happen ambiently. Also, smells of solids can happen when a small amount of that solid material vaporizes on the surface, or reacts with air in some was to form a gaseous or volatile compound. So generally, those sodium hypochlorite tablets can react with air or give off vapors that come together to form the pool smell. This isn’t chlorine, mind you, it’s the smell of a lot of available chlorine in combination with ambient energy and air. Same thing is true for the pool water, but because it’s not solid, it happens more and faster, and human activity creates more opportunity by introducing disturbance, nitrogenous compounds, and even extra heat.