How aggregation functions work?

Hello everyone. I have some misunderstandings about how aggregation functions work for aggregation cells. For example, an example from the documentation (in the screenshot). The library has a number of built-in aggregation functions that can be used for aggregation cells. For example, median, sum, max, etc. Do I understand correctly that if I select some function in aggregationFn in the column definition, for example, "median", then in aggregatedCell the getValue function will return the already calculated value, correct? But if I define my custom aggregation function and use it, then getValue will not return the calculated value to me and I have to manually use this function (getAggregationFn())?