You're seeing just the callback handle_prepare, go back to DBConnection module for more information.
Link to this callback

handle_prepare(query, opts, state)

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handle_prepare(query(), opts :: Keyword.t(), state :: any()) ::
  {:ok, query(), new_state :: any()}
  | {:error | :disconnect, Exception.t(), new_state :: any()}

Prepare a query with the database. Return {:ok, query, state} where query is a query to pass to execute/4 or close/3, {:error, exception, state} to return an error and continue or {:disconnect, exception, state} to return an error and disconnect.

This callback is intended for cases where the state of a connection is needed to prepare a query and/or the query can be saved in the database to call later.

This callback is called in the client process.