"How can we change the items in the Related Products section on each product page? I don’t find any logical reason why products are appearing there at the moment. Thank you!"
According to the WooCommerce documentation, the related products can be influenced by grouping similar products in the same category or by using the same tags. It's an automatic process and there's no direct way to the take control over the list of related items associated with a specific product.
However, you can choose to turn off the Related Products section (Appearance > Customize > Product Page) and set up Cross Selling products instead. Cross Selling items are linked together by you manually and this way you can have full control over it.
Read more in the WooCommerce Docs: