Good day @Andrea Caldarone ,
I've seen no documentation on this topic.
This make sense. Microsoft push for the other direction = migrating to the Azure and not vice versa. But we can guide you on your options.
I've an Azure Managed Instance, my task is to take databases from the cloud and bringing them back online in an on-premise installation.
The solution should fit your needs, and first main question which I would ask is: Do you need the database to be online during the migration or not
Option 1: If you do not care about keeping the database online then backup and restore can fit you. Azure Managed Instance support full backup with COPY_ONLY to Azure blob. For example:
BACKUP DATABASE Your_Database_Name
TO URL = 'https://mytorageaccount.blob.core.windows.net/myfirstcontainer/Your_Database_Name.bak'
WITH STATS = 5, COPY_ONLY;
For more information check the documentation about BACKUP: https://learn.microsoft.com/en-us/sql/t-sql/statements/backup-transact-sql?view=azuresqldb-mi-current#azure-sql-managed-instance
Option 2: you can use replication to on-premises