Why do traditional variable sampling determine the sample size and estimate population standard deviation?

Why traditional variable sampling determines sample size and estimates population standard deviation.

In the traditional variable sampling, we need to stratify the population and determine the sample size of each level. Then sample, evaluate the false positives of each layer respectively, and finally summarize. Look at the size of variability, which can be measured by standard deviation.

Overall variability. The overall variability changes in the same direction as the sample size. It is difficult to determine the sample size by traditional variable sampling.