Azure SQL Database Architecture

There are four layers in Azure SQL Architecture most quite similar to on-premises

Client Layer: Is the interface for applications to access a SQL database the client layer includes SQL Server tools, (ODBC), ADO.NET, (PHP) and Tabular Data Stream (TDS) is used to transfer data between a SQL database and applications. SQL Server also uses TDS to communicate with applications. This allows applications such as .NET, ODBC, ADO.NET, and Java to easily connect to Azure SQL Database without any additional requirements
Service Layer: acts as a gateway between the client and platform layers
Platform Layer: (SQL Server, Azure Fabric, Management Services) Azure Services Fabric is Responsible for Load Balancing , Automatic Failover and Automatic replication of the SQL Databases between Physical Servers https://docs.microsoft.com/en-us/azure/service-fabric/service-fabric-overview and Management Services takes care of an individual server’s health monitoring and patch updates.
Infrastructure Layer: Layer responsible for administrating the OS and physical hardware