Hello,
The Related Products as you know, pulls Products from the same category or tags. So, under the Theme Settings, there isn’t an option to change any sort of logic in this regard because this is the default working of the Related Products section defined by the WooCommerce itself too. You can read out here on the following link about them: https://woocommerce.com/document/related-products-up-sells-and-cross-sells/
Further, if you want to define specific Related Products, then you can edit the Products > in that go to Product Data > click on Linked Products tab > there you can define the Upsells Products which will show on the Related Products area.
Best Regards.