They could, for example, ask for demographic information of the sampled population and weight that to the general population (let’s say that 50% of the people who answered are males from 70+; but they just represent 10% of the general population, then that answers are weighted to represent only 10% of the final results).
There’s a lot more than that, check the sampling methods on this Wikipedia article:
https://en.wikipedia.org/wiki/Sampling_(statistics)
They could, for example, ask for demographic information of the sampled population and weight that to the general population (let’s say that 50% of the people who answered are males from 70+; but they just represent 10% of the general population, then that answers are weighted to represent only 10% of the final results).
There’s a lot of theory behind sampling.
I’m sure it is probably fine. I’m just being flippant.