When you enter your iCloud account (username and password) in Sync Contacts Cloud, the user name and password will be sent to iCloud and nowhere else.
Contacts and events are stored by the respective Android content providers and not by app itself. User data won't be sent to any other server than iCloud.
The app are using "firebase-crash" to tracking app issues (crash, exceptions...). Otherwhile, The app doesn't collect, process or transmit any statistical data like app usage