Получить все товары из подразделов и сгруппировать по родительской категорий

fetchAllProductsFromChildrens.php

<?php

use Bitrix\Main\Loader;
use Bitrix\Main\LoaderException;

const CATALOG_IBLOCK_ID = 17; // ID Инфоблока Каталога

function fetchAllProducts()
{
    try {
        $rootSections = []; // корневые категорий

        Loader::includeModule('iblock');

        $rsParentSection = CIBlockSection::GetList(
            array('ID' => 'ASC'),
            array(
                'IBLOCK_ID' => CATALOG_IBLOCK_ID,
                'DEPTH_LEVEL' => 1,
            ),
            true,
            array(
                'ID',
                'NAME',
                'LEFT_MARGIN',
                'RIGHT_MARGIN',
                'DEPTH_LEVEL',
                'IBLOCK_SECTION_ID',
            ),
            false
        );

        while ($arSection = $rsParentSection->Fetch()) {
            $rootSections[$arSection['ID']] = $arSection;
        }


        $childrenSections = [];
        foreach ($rootSections as $rootSection) {
            $arFilter = array(
                'SECTION_ID' => $rootSection['ID'],
                'ACTIVE' => 'Y',
            );
            $arSelect = array('ID', 'NAME');
            $rsSection = CIBlockSection::GetTreeList($arFilter, $arSelect);
            while ($arSection = $rsSection->Fetch()) {
                $arSection['SECTION_NAME'] = $arSection['NAME'];
                $childrenSections[$rootSection['ID']][] = $arSection;
            }
        }


        $arProducts = [];


        foreach ($childrenSections as $rootSectionId => $childrens) {
            foreach ($childrens as $children) {
                $product = CIBlockElement::GetList(
                    array('SORT' => 'ASC'),
                    array(
                        'SECTION_ID' => $children['ID'],
                        'IBLOCK_ID' => CATALOG_IBLOCK_ID,
                    ),
                    false,
                    false,
                    array(
                        'ID',
                        'IBLOCK_ID',
                        'ACTIVE',
                        'NAME',
                        'CODE',
                        'IBLOCK_SECTION_ID',
                        'IBLOCK_SECTION',
                    )
                );

                while ($arProduct = $product->Fetch()) {
                    $arProduct['ROOT_SECTION_ID'] = $rootSectionId;
                    $arProducts[$rootSectionId]['SECTION_NAME']
                        = $rootSections[$rootSectionId]['NAME'];
                    $arProducts[$rootSectionId]['PRODUCTS'][] = $arProduct;

                }
            }
        }

        print_r($arProducts);
    } catch (LoaderException $exception) {
        print_r($exception->getMessage());
    }
}