I'm sure others will be able to give better detail but, basically, the Server (Tableau Server) sends the SQL request to the data source db, which executes the SQL, and sends the results to the Server which crunches the data into the extract. So, yes, your data source db naturally will be doing some work.
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The SQL runs against the view on the database via an internal Tableau process (tabprotosrv) which is the main communicator between Tableau and the database via the ODBC driver.
Then there is communication of data across and a Tableau process called tdeserver64 handles importing the data and generating the extract.