How do they measure the "__% of Americans" surveys?

You always hear statistics like "73% of Americans believe XYZ..." I don't remember ever providing my answer to such a survey, though.

I remember reading somewhere how they get these numbers. Something like surveying a small population.

But there is no way that is accurate, right? If we has a single population, or even a couple of small populations, their opinion on something, that leaves so much room for the answers from the rest of the country empty. How can we base the entire country on a small population? Do knowledgeable people agree those statistics are BS, or do they actually hold weight?