How does Tableau know at which level to aggregate values?

In Tableau, you can aggregate measures or dimensions, though it is more common to aggregate

measures. Whenever you add a measure to your view, an aggregation is applied to that measure by

default. The type of aggregation applied varies depending on the context of the view.

When you add a measure to the view, Tableau automatically aggregates its values. Sum, average, and

median are common aggregations; for a complete list, see List of Predefined Aggregations in Tableau.

The current aggregation appears as part of the measure's name in the view. For example, Sales

becomes SUM(Sales). Every measure has a default aggregation which is set by Tableau when you

connect to a data source. You can view or change the default aggregation for a measure—see Set the

Default Aggregation for a Measure.

You can change the aggregation for a measure in the view from its context menu:

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Reference: https://help.tableau.com/current/pro/desktop/en-us/calculations_aggregation.htm