QuickBooksPOS to ShopifyPOS - Where is QB ListID being used and merging variant listings?

[AGNDawn]

I've been all over Google but still can't quite figure this one out.  I'm building a site for a customer, they have about 3000 products that were imported from QuickBooksPOS to ShopifyPOS.  Client is still using the QuickBooks Desktop Accounting Software to run the day to day within their single-location brick and mortar.

Client is looking to simplify the online-store with this conversion and take all of the like-products and merge them to one listing with variants.  Right now, for example, site has 12 listings for a single red shirt on the backend as each has a unique barcode for their sizing (Mens/Womens S/M/L/XL etc). It would seem to be as simple as taking one listing and pushing all of the barcodes into it, but with the QuickBooks export there's a secondary QBListID required to sync the inventory, I think?  Where exactly do these ListID's store?  I've got the ShopifyPOS app and the QuickBooks app and Stocky, neither show me where that ID is being stored so there's no confirmation that I can merge all of these variant listings without something breaking within QuickBooks/Shopify sync.

How should I go about this where his inventory numbers will not be an issue, but he can still have 12 barcodes brought over to one listing with the QBListID still intact?  Does a certain listing need to be the primary for the variants to be added to?  All listings have a unique QBListID so I don't know where to start.

Thank you in advance!