DECLARE @Json nvarchar(MAX) = (
    SELECT name, surname  
    FROM emp  
    FOR JSON AUTO
);

Dan Guzman replied in the MSDN Forum with this neat solution, which corresponds also to @FDavidov's suggestion in his last comment

A JSON is, in fact, a character string. What makes this character string to be a JSON is the combination of two things:

1. You refer to it as a JSON (using the correct functions within your environment),
2. It contains the correct delimiters at the correct locations to comply with the rules of a JSON.

So, if you want to store a JSON in a variable, just assign to the variable the source string and, whenever you want to act on your variable, just remember it is a JSON.

I was looking for a similar thing, but how to use the JSON PATH output data from SQL Server in an application. In this case, in Node.JS (although this concept would be adaptable to other languages). Here's a sample of what I came up with:

var {recordset} = await new sql.Request(primary.dbPool)
.input('primaryID', sql.UniqueIdentifier, primaryID)
.query(`SELECT (
    SELECT MainTable.*,
        (
            SELECT SubTable.* 
            FROM dbo.SubTable AS SubTable WHERE MainTable.Main_RecordID = SubTable.SubTable_Main_RecordID 
            FOR JSON PATH
        ) AS [subItems]
    FROM dbo.MainTable AS MainTable
    WHERE primaryID = @primaryID
    FOR JSON PATH, ROOT('mainItems')
) AS jsonResult;`)
let mainItems = JSON.parse(recordset[0].jsonResult).mainItems

console.log(mainItems)

This avoids it being returned as a long string in a UUID field like:

And is instead something you can use in the application.