Products on Ordentic sit in a shared catalog with your commercial listing on top. The same product is identified by manufacturer reference and brand. That matching key is how offers from different suppliers land on one product page for buyers.
Import and listing flows can also carry fields such as GTIN and your own reference. Those help you manage your assortment. They are not the primary identity for matching products across suppliers.
Keep manufacturer reference and brand accurate when you create or update listings in the portal, via import, or through the Supplier API: