Drupal Commerce Line Items
Judging from your question you seem very confused about line items and what they are used for.

Line items are used to link products to an order. The whole idea is that some data you want to fetch from the product directly, like it's title, media etc, while some data you need to copy, like the price and sometimes you need to add data, like the address for a subscription and quantity.

The reason you need to copy the price, is that if you ever change the price of the product you want to preserve the old orders which is why you need to store the price for each purchase.

There is nothing technical that will hinder you to create a relationship from products to line items or from line items to line items, but it won't make much sense to do so, and will most likely result in some strange bugs, as line items are used for orders to store info about products, and commerce expects this.