Ash.DataLayer.upsert

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

upsert(resource, changeset, keys)

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Specs

upsert(Ash.Resource.t(), Ash.Changeset.t(), [atom()]) ::
  {:ok, Ash.Resource.record()} | {:error, term()}