Thanks for reaching out! There are a couple ways to create extracts - more information can be found in the Product Help.
- Selecting the Extract radio button when first connecting to your data source in Tableau Desktop.
- Selecting the Extract Data option from the context menu, while on a worksheet.
Note: Workbooks that use published data sources will display the connection in the workbook as LIVE on Tableau Online - this is sometimes confusing since an extract was published. In this scenario, the LIVE connection indicates the workbook is connected "live" to the extract hosted on Tableau Online.
Hope this helps!