Tableau extract will be created after SQL join execution.
That means once you define your Inner join with required filter, in background it will create a SQL code to fetch data from source then resultant data will be stored in your .tde file.(This is for same data source connection).
Newer version comes with Inter datasource join. In that case Tableau data engine fetch data from different datasource to its temporary area the apply join condition to store only required data in .tde file.
Hope this helps.
Thanks Sandeep for the prompt reply, this information is helpful to me.
One more query , Can you please tell me in which version we can Join two/multiple Datasources.
here is the article to help you out. I have not tried yet but afterwards 10 version is there what i have heard.