When configuring product categories for your store, you may want your categories to display not only the products within the category, but also products contained within any subcategories of that category.
Do this by enabling Display Subcategory Products on Main Category Page in category settings:
Log in to your Admin Area and go to Inventory > Categories.
Click View List.
Click the three-dot actions menu icon and select Categories & Subcategories Settings.
Select Display Subcategory Products on Main Category Page.
Once this setting has been activated, your storefront will display all products within each of its main category pages whether the product is a member of the category or a member of the category's subcategories.