TurboTax Online does not have all the same features of the CD/Download ("desktop") TurboTax software. It makes no difference when you pay for it. (What might be causing some misunderstanding is that you have to pay up front for the CD/Download software.)
The most significant feature in the CD/Download software that TurboTax Online does not have is forms mode. There is no way to see the actual tax forms except by creating a PDF of the return, and there is no way to make entries directly on the forms. All entries have to be made in the interview. The lack of the forms mode also means that certain other capabilities are not available, such as Data Source and QuickZoom to trace the data flow and see how numbers in the return were calculated. Those capabilities are what let you "drill down" to see where a number came from.
However, TurboTax Online does have a few features that the CD/Download software doesn't have, such as the ability to import transactions from some crypto exchanges.
