As I understand it, if you get the virus whilst it can multiply within your body, it is also being killed by your body's defences. (And if you are lucky, you end up killing all of the viruses.) The dead virus breaks down and is eliminated in presumably your urine and faeces (as is the live virus ? so be careful cleaning other persons back sides). Parts of the RNA within the virus has specific ?sequences? which uniquely define the virus. The broken down dead virus thus releases these identifiable parts into the effluent system, through sewage plants and hence into the environment, usually in rivers.
Sampling rivers or indeed the outfall from sewage plants can then detect these identifiable parts. The more people with the virus, the higher the concentration. But it is far from a one to one relationship and depends on many other factors, the amount of rainfall, the amount of dilution by water usage in the home to name but two significant factors in the dilution effect. I suspect predicting what proportion of the population has it is may be a step too far, but it does provide a useful indicator of where the virus has almost certainly been. Hence the recent repeated claim that Italy had it late last year when no one was reporting the symptoms.