Using Copy Activity's advanced editor you can map the JSON value to their DB field as shown below

It will load the data as below

Now you can apply the JSON_VALUE() function to parse the Userjson using below query

SELECT 
[id],body,postid,likes
,JSON_VALUE(userjson,'$.id') AS UserID
,JSON_VALUE(userjson,'$.username') AS UserName
,JSON_VALUE(userjson,'$.fullName') AS fullName
FROM [dbo].[TestJson] as t

It will give the below result