Q: I'm using Azure AD. Where the data is stored? Also, if I have employees geographically distributed, will they access an Azure data center closest to them for Azure AD communications?
A: Communications with Azure AD are actually very light and based on web protocols that are designed to handle high latencies, which means if your Azure AD is primarily stored in the US and users are in Europe, there shouldn't be a problem. However, Azure AD actually works as a system of an active primary, a passive primary, and then a number of secondary replicas that are replicated around the world. When writes are performed, they're performed at the active primary then replicated, whereas reads can be performed against the primary or secondary replicas, enabling distributed communication throughout the world.
For more details, see the Active Directory Team blog post "Azure AD: Under the hood of our geo-redundant, highly available, distributed cloud directory."