Commerce: How to manage line item fields for custom product type? Drupal commerce works as follows:

When I add a product to the cart, system creates new line item (of type: product) 

I have several product types. Could I achieve, that product line items would be specific for each product type? 

What I would like to achieve is, that I can manage fields in line item type separately for each product type. 
For now, I can manage fields (or manage displays) only generally for all product types (When I go to Store -> Configurations -> Line item types -> Product)
There's an open feature request for precisely what you want in the queue for Commerce Customizable Products. In the meantime, you need to create a new product display node type for each product type and use the settings for the Add to Cart form display formatter to use the pertinent line item type in its forms.