When you have two CSV files you want to join then you can do that here.

 

In the case that you want to find out more about the CSV format you can go to the chapter Output settings.

 

Go to the menu point Tools > Join CSV files:

 

 

​On the left side you can upload the first file by clicking on the three dots behind CSV file.

 

 

You can read about how to upload a CSV file in the chapter Upload files.