Ecto.Query.API.merge

You're seeing just the function merge, go back to Ecto.Query.API module for more information.
Link to this function

merge(left_map, right_map)

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Merges the map on the right over the map on the left.

If the map on the left side is a struct, Ecto will check all of the field on the right previously exist on the left before merging.

from(city in City, select: merge(city, %{virtual_field: "some_value"}))

This function is primarily used by Ecto.Query.select_merge/3 to merge different select clauses.