Blank Product

There's no built in native way in shopify to link different products inventories together.

So this product is used to represent a prime product that in turn represents the real inventory for a set of related products, aka product family aka product siblings.

This product is intended to be swapped in before checkout replacing other line items , with this product having line-item-properties defining the values of the swapped-out product or a reference to it like that products handle or variant id, or options, etc.

Because shopify does not have a concept of inventory pooling, or inventory-linking etc etc ,  imagine the following scenario:

You want to make custom t-shirts that you screenprint art onto blank tshirts of which you have 10.

You want to have a product listing for specific art: product A and product B. 

So you give A 10 inventory and B 10 inventory so at least you never sell more than 10 of each.

So you give A 10 inventory and B 10 inventory so individually each sells no more than 10 because after all you can make at least 10 shirts.

But someone could by all of A and B for 20 shirts!

10 of which you can't make because you actually only have 10 blank t-shirts.

Normally to have inventory control each product get's it's own inventory quantity but if you only have 10 blank tshirts, you cannot actually sell any combination of more than 10 quantity of product A and product B.

With no built in way to reference one products inventory to anothers a workaround is to simply use ONE product as the source of truth for inventory.

But that means it's that product that must go to checkout and be part of the order.

So how do you sell other products that then become this prime product.

At least through theme customizations you use javascript, and or form redirects to swap this product in for the other products it's meant to replace.

An alternative to test is if bundle apps can add free products only to a bundle with the prime product. In that scenario the free product represents the customers art choice , options selections, or other. But then the trick is making sure those "free" products cannot be added to the cart by themselves, or really even viewed as "free" without pricing to avoid confusing customers.