I have switched to Iphone 14 pro max from iphone 12 pro max, after transferring my data I am unable to open or download the inbuilt apps from Apple such as Mail, contacts, calendar, clock.

Salutations Ali-Hamza-Essa, 

Please be sure that the region you changed, whether it be for your device or your Apple ID, has been set to the correct region. 

Next, If there is any security software or VPN software that was installed on your previous device and has now been moved over, please remove it, restart your iPhone and test again. Check VPN (Virtual Private Network) and third-party software to help resolve network connectivity issues

If the issue is persisting, can you tell us whether it says anything like ‘Waiting’, ‘Paused’, or something else under those apps? 

Thanks.