How is Bigdata handled, RDBMS or NoSQL?

 Here is a reasonable article comparing SQL vs. NoSQL. Here you can also look up the differences between RDBMS and Document Databases. 

https://phoenixnap.com/kb/sql-vs-nosql

While Amazon has its own DocumentDB, MongoDB is used in a lot of places (Forbes, Toyota, etc) and Amazon's DocumentDB is compatible with MongoDB. 

Of course, Microsoft's SQL Server is a mature product and it can even handle BigData using Polybase virtualization. You can query data from any SQL Server, Oracle, Teradata, MongoDB, and other data sources using external tables. 

https://docs.microsoft.com/en-us/sql/big-data-cluster/big-data-options?view=sql-server-ver16

Connectivity to HDFS now uses published REST APIs instead of the Java Hadoop client. all you need to do is to configure connectors while configuring the AZURE Storage.

Here is a schematic from Microsoft's documentation of BigData storage and processing in the Microsoft platform.

Also, storing data in itself is not sufficient and Microsoft has POWER BI which also visualization of data from a huge number of database products. It is hard to beat Microsoft at this game.

I am somewhat slanted towards Microsoft due to my association with Microsoft database products for a long time. I have not received any remuneration from Microsoft for this post.

How do you obtain JSON formatted response to a SQL Query?

There are two ways you can do this as per my experience, but there could be more ways.
One method I found was to install the DocumentDB Migration tool from Microsoft and launch the program to convert the SQL Query response to a JSON format.

The details of the migration tool and downloading the tool are described here:

http://hodentek.blogspot.com/2015/05/tips-and-trick-to-format-sql-reponse-in.html

A step-by-step description of how it is done is described here:

http://hodentekmsss.blogspot.com/2015/05/migarting-from-sql-to-json-using.html

The second method is to download a software tool called SQL-JSON from the internet and use the tool. This tool can run both on Windows and Apple Mac OS. It can be used with SQL Server 2008 and SQL Server 2012 and requires Java Runtime 6 or 7.

The download link is here:
http://www.sqljson.com/index.html