You're seeing just the function aggregate, go back to Ash.Query module for more information.
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aggregate(query, name, type, relationship, agg_query \\ nil, default \\ nil, filterable? \\ true)

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Adds an aggregation to the query.

Aggregations are made available on the aggregates field of the records returned

The filter option accepts either a filter or a keyword list of options to supply to build a limiting query for that aggregate. See the DSL docs for each aggregate type in Ash.Resource.Dsl for more information.