Access Customizer > WooCommerce > Product page – Content and here, look for Related products – Custom products option and enable it eg: https://d.pr/i/t3AHiJ .
Enabling this option will add a custom text input into the products pages in product backend, in the Linked products tab, to be able to select custom products.
You can also enable replacing the products rather than just appending the products to the already existing list.
After enabling the Custom products option, you can now access the product backend and select the products you want to display:
