Hello,
To display the related products section 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.
Also you can read and follow the below documentation for in depth guidance: https://docs.woocommerce.com/document/related-products-up-sells-and-cross-sells/
Best Regards.
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