When you create a new Calendar event from iStudiez Pro on your iPhone/iPad, it is created in Calendar app, not in iStudiez Pro.
iStudiez Pro can sync the data that contains the app itself (e.g. semesters, courses, assignments, etc.).
This data is synced via iStudiez Pro Cloud Sync Server.
Calendar events are synced via iCloud, the service that cannot be used by iStudiez Pro.
Apple provides iCloud API to third-party developers for macOS and iOS only. As of now it is not available for Windows.
The Windows version of iStudiez Pro can show Google Calendar events.
Thus, to see Apple Calendar events in iStudiez Pro on Windows you should sync Apple Calendar with Google Calendar and/or vice versa.
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