Hello,
Products of related category will be shown on the single product page. In your screenshot you pointing out the WordPress custom page which contains product tabs and inside it, products from different categories exists
To display the related products section in your single product page you need to enable the “Show related products” option from Theme Settings >> Product Page >> SHOW / HIDE ELEMENTS.
Then you have to select a category from the product edit page as in the screenshot: https://jmp.sh/yqABzGC
For example, if you have three products and all of them are of the same category then in the related products section of all those three products the other remaining 2 products will be shown.
Best Regards.