How to Construct Nested JSON Message on Output Topic in KSQLDB

It's not terribly clear what you're trying to achieve, given that your example SQL won't produce the example output, given then example input. In fact your example SQL would fail with unknown column errors.

Something like the following would generate your example output:

CREATE TABLE EVENT_TABLE AS 
  SELECT 
    status,
    avg(eventDetails->minpayment) as Avg_MinPayment, 
    avg(eventDetails->apr) AS Avg_APr, 
    avg(eventDetails->offeramount) AS Avgofferamount
  FROM STREAM1 
  GROUP BY status 
  EMIT CHANGES;

Next, your example output...

Status | Avg_MinPayment | Avg_APr | Avgofferamount 
-----------------------------------------
Approved | 400 | 2.6% | 40000

Rejected | 600 | 3.6% | 70000

...is outputting one row per status. Yet, the output you say you want to achieve ...

{
 "event":{
  "header":{
   "name":"abc",
   "version":"1.0",
   "producer":"123",
   "channel":"lab",
   "countryCode":"US"
  },
  "body":{
   "Key":[
{"Status":"approved","Avg_Minpayment":"400","Avg_APr":"2.6%","offeramount":"40000"},
{"Status":"rejected","Avg_Minpayment":"600","Avg_APr":"3.6%","offeramount":"70000"}
  ]
 }
}

...contains both statuses, i.e. its combining both of your example input messages into a single output.

If I'm understanding you correctly, and you do indeed want to output the above JSON, then:

You would first need to include the event information. But which event information? If you know they're always going to be the same, then you can use:

CREATE TABLE EVENT_TABLE AS 
  SELECT 
    status,
    latest_by_offset(event) as event,
    avg(eventDetails->minpayment) as Avg_MinPayment, 
    avg(eventDetails->apr) AS Avg_APr, 
    avg(eventDetails->offeramount) AS Avgofferamount
  FROM STREAM1 
  GROUP BY status 
  EMIT CHANGES;

The latest_by_offset aggregate function will capture the event information from the last message it saw. Though I'm not convinced this is what you want. Could you not be getting other rejected and accepted messages with different event information? If it is the event information that identifies which messages should be grouped together, then something like this might give you something close to what you want:

CREATE TABLE EVENT_TABLE AS 
  SELECT 
    event,
    collect_list(eventDetails) as body
  FROM STREAM1 
  GROUP BY event 
  EMIT CHANGES;

If this is close, then you may want to use the STRUCT constructor and AS_VALUE function to restructure your output. For example:

CREATE TABLE EVENT_TABLE AS 
  SELECT 
    event as key,
    AS_VALUE(event) as event,
    STRUCT(
      keys := collect_list(eventDetails)
    ) as body
  FROM STREAM1 
  GROUP BY event 
  EMIT CHANGES;