Oracle 18c

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CREATE TABLE myjson( doc VARCHAR2(4000) check (doc is JSON) );
INSERT INTO myjson VALUES('{
  "totalResultsCount": 19,
  "geonames": [
    {
      "geonameId": 2593109,
      "fcode": "ADM1"
    },
    {
      "geonameId": 3336899,
      "fcode": "ADM1"
    }]
}    
');         
1 rows affected

   SELECT ROW_NUMBER() OVER (ORDER BY 1) - 1 AS row_id, js.product 
     FROM myjson
    CROSS JOIN JSON_TABLE( 
                          doc, '$.geonames[*]' COLUMNS (
                                               product VARCHAR(100) PATH '$.geonameId',
                                               fcode   VARCHAR(100) PATH '$.fcode'
                                               )
                ) js
    WHERE js.fcode = 'ADM1' 
    


ROW_ID
PRODUCT


0
2593109

1
3336899


DECLARE
  json_array_t OWA.VC_ARR;
BEGIN
  FOR c IN
  (
   SELECT ROW_NUMBER() OVER (ORDER BY 1) - 1 AS row_id, js.product 
     FROM myjson
    CROSS JOIN JSON_TABLE( 
                          doc, '$.geonames[*]' COLUMNS (
                                               product VARCHAR(100) PATH '$.geonameId',
                                               fcode   VARCHAR(100) PATH '$.fcode'
                                               )
                ) js
    WHERE js.fcode = 'ADM1' )
  LOOP    
    json_array_t(c.row_id) := c.product;
    DBMS_OUTPUT.PUT_LINE( 'value for json_array_t('||c.row_id||') is '||json_array_t(c.row_id) );
  END LOOP;
END;
/
1 rows affected

dbms_output:
value for json_array_t(0) is 2593109
value for json_array_t(1) is 3336899