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 tbl1 (val1 varchar2(10), val2 varchar2(10), val3 varchar2(10));
BEGIN
INSERT INTO tbl1 VALUES ('1','2','3');
INSERT INTO tbl1 VALUES ('a','b','c');
INSERT INTO tbl1 VALUES ('1','b','3');
INSERT INTO tbl1 VALUES ('2','d',NULL);
END;
/
1 rows affected

CREATE FUNCTION object_to_array(
  value IN VARCHAR2
) RETURN VARCHAR2
IS
  js   JSON_OBJECT_T := JSON_OBJECT_T( value );
  arr  JSON_ARRAY_T := JSON_ARRAY_T();
  keys JSON_KEY_LIST := js.get_keys();
BEGIN
  FOR i in 1 .. keys.COUNT LOOP
    arr.append(js.get_string(keys(i)));
  END LOOP;
  RETURN arr.to_string();
END;
/
SELECT * FROM USER_ERRORS;
SELECT json_object(
         KEY 'keys' VALUE MIN(p.keys) FORMAT JSON,
         KEY 'values' VALUE JSON_ARRAYAGG(object_to_array(t.json) FORMAT JSON)
       ) as js
FROM   (SELECT JSON_OBJECT(*) AS json FROM tbl1) t
       CROSS JOIN LATERAL(
         SELECT JSON_DATAGUIDE(t.json) AS data FROM DUAL
       ) d
       CROSS JOIN LATERAL(
         SELECT JSON_ARRAYAGG(SUBSTR(path, 3)) AS keys
         FROM   JSON_TABLE(
                  d.data,
                  '$[*]'
                  COLUMNS(
                    path VARCHAR2(20) PATH '$."o:path"'
                  )
                )
         WHERE  path != '$'
       ) p


JS


{"keys":["VAL1","VAL2","VAL3"],"values":[["1","2","3"],["a","b","c"],["1","b","3"],["2","d",null]]}