Population variance known vs. unknown

How do we know when the population variance is known or unknown without being told if it is known or unknown? So far we have been told in the title of the lessons whether or not variance is known or unknown, and Excel has functions for sample variance and population variance so with either function we should be able to find the variance right? Logically if that's true then variance will always be known when using those functions. We would just have to know if the data were sample or population data to use the correct variance function.

In both the z-score and t-score lessons we calculated the Mean, St Dev, and SE. The z-score lesson had a larger dataset but so far that seems to be the biggest difference between the two lessons. I understand the z-score lesson needed to have a known population variance so you could teach it but I don't know what that variance is. Since the t-score lesson also had the Mean, St Dev, and SE the information required seems identical yet the t-score has an unknown population variance.