You're seeing just the function select, go back to Ash.Query module for more information.
Link to this function

select(query, fields, opts \\ [])

View Source

Ensure that only the specified attributes are present in the results.

The first call to select/2 will replace the default behavior of selecting all attributes. Subsequent calls to select/2 will combine the provided fields unless the replace? option is provided with a value of true.

If a field has been deselected, selecting it again will override that (because a single list of fields is tracked for selection)

Primary key attributes are always selected and cannot be deselected.

When attempting to load a relationship (or manage it with Ash.Changeset.manage_relationship/3), if the source field is not selected on the query/provided data an error will be produced. If loading a relationship with a query, an error is produced if the query does not select the destination field of the relationship.

Use ensure_selected/2 if you simply wish to make sure a field has been selected, without deselecting any other fields.