Oracle 18c

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CREATE TABLE myjson (
  doc   CLOB,
  CONSTRAINT json_documents_json_chk CHECK (doc IS JSON)
);
INSERT INTO myjson VALUES 
('{
  "totalResultsCount": 19,   
   "geonames":[
    {
      "geonameId":2593109,
      "fcode": "ADM1"
    },
    {
      "geonameId": 3336899,
      "fcode": "ADM1"
    }          
    ]
}');
1 rows affected

DECLARE
  l_str          VARCHAR2(32767);
  l_top_obj      JSON_OBJECT_T;
  l_dept_arr     JSON_ARRAY_T;
  l_dept_obj     JSON_OBJECT_T;
  l_emp_arr      JSON_ARRAY_T;
  l_emp_obj      JSON_OBJECT_T;
BEGIN
  SELECT j.doc 
    INTO l_str
    FROM myjson j 
   WHERE j.doc.geonames.fcode LIKE '%ADM1%';    
  
  l_top_obj := JSON_OBJECT_T(l_str);
  l_dept_arr := l_top_obj.get_array('geonames');
  
  FOR i IN 0 .. l_dept_arr.get_size - 1 
  LOOP
    l_dept_obj := TREAT(l_dept_arr.get(i) AS JSON_OBJECT_T);
    DBMS_OUTPUT.PUT_LINE('index  : '||i||' - geonameId : ' || l_dept_obj.get_number('geonameId'));
  END LOOP;
END;
/
1 rows affected

dbms_output:
index  : 0 - geonameId : 2593109
index  : 1 - geonameId : 3336899