How Related Products works
Related products are automatically added to product pages. It's WooCommerce who controls this functionality.
Related products is a section on some teup sells that pulls other products from your store that share the same tags or categories as the current product. These products can not be specified in the admin, but can be influenced by grouping similar products in the same category or by using the same tags.
There is options in Theme Options > Product page for related products.