In order to buy public transport tickets in Stuttgart, you need the SSB BestPreis app. You can search for a public transport connection in the moovel app and click on "Open" to go directly to the same connection overview in the SSB BestPreis App. There you can then buy a ticket for the chosen connection - at the best price for you.
The SSB BestPreis app must be installed on your smartphone. You can download it from the App Store or Google Play Store. However, you do not have to create an additional account for the SSB BestPreis app - your moovel account and payment method can also be used here.