I'm confused what a "sample" group means in this context. Take people who have one and only one sibling: For the studied population (people who chose to take

I'm confused what a "sample" group means in this context. Take people who have one and only one sibling: For the studied population (people who chose to take the SSC survey) the % of people who were the older sibling was some number. The "sample" to compare that to is...what? The human population as a whole? Then we don't need a sample group, we know it has to be 50%. So if the number he got in his study is much different from 50% that says something about the population in the study being different.