SCA is a cloud native App, which gets all contact information from your Azure Tenant. When SCA launches for the first time or the user performs the pull-to-update gesture, the resync process is started. During this resync process, SCA queries all configured data sources. Only data sources are accessible, to which the user is authorized.
The Primary data sources are the Azure Active Directory (AAD) and the Global Address List (GAL). Additionally it is possible to get contact information from the users personal Outlook Contacts (APC), this requires Exchange Online.
Optional data sources need additional configuration in your Azure Tenant. These data source are Dynamics 365 (D365), MS Dataverse (DVRS), Azure Blob Storage (ABS) and Shared Mailbox Contacts (SMC).