There are two Azure SQL PaaS offerings: Azure SQL Database and Azure SQL MI (Managed Instance).
Azure SQL Database is a single database; whereas Azure SQL MI is closer to a normal on-prem Instance (although not exactly the same).
Both come with their own AlwaysOn AG (automatically created and managed for you). This is the foundation of Azure's high availability architecture in Azure SQL Database and SQL Managed Instance. I don't think it works, from an architecture point-of-view, for an on-prem AG to have a replica in Azure, which is itself part of another AG. Never mind the requirements for a WSFC.