If you want statistics that make a minimum of sense, you need a representative sample. "Anyone who was willing to respond" is not representative, which is why any internet survey where anyone can respond is garbage.

To create a representative sample, you need knowledge on your population. For survey on the general population of a given country or state, the census data is used. You know what percentage of your population is in a given age bracket, their occupation, their gender, their family situation, etc. and you build a sample with the same proportions.