Odoo v 14

When creating a product, the Internal ID is mapped to "default_code" in product_template database table. If I edit the product, there is no "Internal Id" field available, and when I hit the "save" button this field is removed/blanked out. Saving an updated product deletes the default_code field (internal id). 

However, if I go to each variation I can enter the "Internal Id" for the variation and it saves on update. 

It seems that nothing shows up in "alternative products" or "accessory products" which does not have an internal ID. 

What I would like is to have "recommended products" show up on the product sales page. There's a block i can drop in which shows products in an entire category, but I don't want that. I want to specify the products that show up in "recommended products".

I'm not sure what "Accessory products" or "Alternative products" are, since they only show products with "Internal Id". None of the products I want appear in "Alternative products" but the variations with Internal ID show up in "Accessory products". But actually I don't want to list each variation. I just want the main product to show up as recommended. 

So I guess the question/problem is:

1) why is internal id on the main product deleted on update

2) what are accessory / alternative for? They don't show up anywhere on the product page that i can see.

3) what do people do to list "recommended products" on the product page?

Thanks