My question is how is the SQL database exposed for an interaction with for example an apex HTTP callout?
The easiest way – today – to make Azure SQL reachable via HTTP is using Azure Function Bindings and create a simple Azure Function that allows an HTTP request to be received and then execute CRUD operation on the database. You can apply one or more output bindings to an Azure Function with an HTTP trigger to insert/update data in an Azure SQL Database.
https://learn.microsoft.com/en-us/azure/azure-functions/functions-bindings-azure-sql
Hope this answers your question.
Regards,
Oury