Magento – how to change root category to subcategory

In Magento you can only have one root category per store.

If you are unfamiliar with Magento's Global->Website->Store->Storeview Scope structure , try reading something like this: Magento Multi Store, Websites and Storeviews Explained or this:How to set multiple websites with multiple store views in Magento

Long story short. In the front-end you cannot have two Root Categories shown simultaneously. Make two Level-1 categories Root->Default and Root->Collections and move the respective subcategories in there.

You can use like this:

/** \Magento\Catalog\Api\CategoryRepositoryInterface */
protected $categoryRepository;

/** \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory */
protected $productCollectionFactory;

public function getAllProductOfSubcategories($categoryId, $storeId = null) {

    /** @var $result \Magento\Catalog\Model\ResourceModel\Product\Collection */
    $result = $this->productCollectionFactory->create();

    //get category at $storeId 
    try {
        $category = $this->categoryRepository->get($categoryId, $storeId);
    } catch (\Magento\Framework\Exception\NoSuchEntityException $noSuchEntityException) {
        return null;
    }

    return $result->addCategoryFilter($category);
}

*Note: Your category must Enable Anchor

*Note: run php bin/magento indexer:reindex just for make sure