Hi @cihatcetintr,
Related products may not work properly due to the template you’re using so you’ll want to test with other themes (Storefront, TWenty Twenty, etc.). Normally, related products feature pull products from your store that share the same tags or categories as the current product.
It is important to note that if you have a large WooCommerce store with lots of categories and subcategories, the related products area will randomly select products from not only the same category but any parent category above the category set for the currently displayed product.
If you want to only show related products from the same category or sub-category try the snippet on this article. Make sure to save a full backup before you do any changes to your site.
All the best.