JSFiddle - React, Tailwind, and code Playground

JavaScript

const productList = [{
	productGroup : 'Namkeen',
  index: 1,
  items: [{
  	name: 'Haldiram',
    item1: 'Bhujia',
    item2: 'Aloo Bhujia'
  },{
  	name: 'BiKaji',
    item1: 'Bhujia',
    item2: 'Aloo Bhujia'
  },{
  	name: 'Bikaner',
    item1: 'Bhujia',
    item2: 'Aloo Bhujia'
  }]
}, {
	productGroup : 'Sweets',
  index: 2,
  items: [{
  	name: 'Haldiram',
    item1: 'Chole Bhature',
    item2: 'Barfi'
  },{
  	name: 'Om sweets',
    item1: 'Raj Kachori',
    item2: 'Gazar Halwa'
  }]
}];

const itemList = productList.map(obj => {
	obj.items.map((itemObj, index) => {
  	itemObj.productGroup = obj.productGroup;
    itemObj.pgIndex = obj.index;
    itemObj.itemIndex = obj.index + '' + index;
  });
  return obj.items;
});

var mergedListItems = itemList.reduce((arr, arrNext) => arr.concat(arrNext));

console.log('mergedListItems', mergedListItems);