Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE sub(dept,phone,device,grp) AS
  SELECT 'STAFF' ,'1234567890','google' ,1 FROM dual UNION
  SELECT 'STAFF' ,'0004567890','apple' ,1 FROM dual UNION
  SELECT 'CEO' ,'0094567890','google' ,1 FROM dual UNION
  SELECT 'CEO' ,'0055567890','google' ,1 FROM dual UNION
  SELECT 'MGR' ,'0085456789','google' ,2 FROM dual UNION
  SELECT 'MGR' ,'0704545478','google' ,2 FROM dual UNION
  SELECT 'MGR' ,'0654565460','apple' ,2 FROM dual UNION
  SELECT 'EXEC' ,'0084467890','apple' ,2 FROM dual UNION
  SELECT 'TRAINEE' ,'1234770000','apple' ,3 FROM dual UNION
  SELECT 'TRAINEE' ,'0354430000','apple' ,3 FROM dual UNION
  SELECT 'TRAINEE' ,'0067556557','google' ,3 FROM dual
11 rows affected

SELECT JSON_OBJECTAGG(
         KEY 'grp'||grp VALUE JSON_OBJECTAGG(
           KEY device VALUE device_json
           RETURNING CLOB PRETTY
         )
         RETURNING CLOB PRETTY
       ) AS result 
FROM   (
  SELECT grp,
         device,
         JSON_ARRAYAGG(
           JSON_OBJECT(
             KEY 'dept'  VALUE dept,
             KEY 'sound' VALUE 'default.caf',
             KEY 'token' VALUE phones
             RETURNING CLOB
           )
           RETURNING CLOB PRETTY
         ) AS device_json
  FROM   (
    SELECT grp,
           device,
           dept,
           JSON_ARRAYAGG(phone) AS phones
    FROM   sub  
    GROUP BY grp, device, dept
  )
  GROUP BY grp, device
)
GROUP BY grp;


RESULT


{
  "grp1" : {
  "apple" : [
  {"dept":"STAFF","sound":"default.caf","token":["0004567890"]}
],
  "google" : [
  {"dept":"CEO","sound":"default.caf","token":["0094567890","0055567890"]},
  {"dept":"STAFF","sound":"default.caf","token":["1234567890"]}
]
},
  "grp2" : {
  "apple" : [
  {"dept":"EXEC","sound":"default.caf","token":["0084467890"]},
  {"dept":"MGR","sound":"default.caf","token":["0654565460"]}
],
  "google" : [
  {"dept":"MGR","sound":"default.caf","token":["0085456789","0704545478"]}
]
},
  "grp3" : {
  "apple" : [
  {"dept":"TRAINEE","sound":"default.caf","token":["1234770000","0354430000"]}
],
  "google" : [
  {"dept":"TRAINEE","sound":"default.caf","token":["0067556557"]}
]
}
}