Correct, there is no marketplace in the SmartThings app, only in the SmartThings Classic app. You can install and use both apps on your phone. When you install the SmartThings Classic app, just click on New to SmartThings when you login and use your Samsung account.
If the app requires ST Classic to install it, once you do, you should be able to access it from the SmartThings app. Two places to look - click on the three bars in the upper left and choose Automations or in settings under connected services.