Calculating sample proportion variance from binary outcomes

I’m confused about calculating sample proportion variance when dealing with binary outcomes when you know the proportion in the population.

Say we have a population of size N where p of them are male. Then if we were to take a sample of n of those people, how could you work out the variance of the proportion that are male in the sample?

I have looked at using estimators, but can’t see how I would relate them together since there’s no variance in the population as we already know p.

Thank you