How to find Google Calendar app on my MacBook after downloading with iTunes?
App Store in iTunes is for iOS apps, i.e. iPhone, iPad and iPod touch.

For Mac apps, you can download these from the Mac App Store: the App Store.app in the Applications folder. Mac apps can also be downloaded from anywhere else on the internet.

There is no Google Calendar app made by Google for macOS. You can use the built-in Calendar.app to access your Google Calendar. Is there a Google Calendar Mac Desktop App?