When products are created directly in the Shopify Admin (not within the app/portal) without the Supplier assigned, you can filter for these products and assign them within the app.
Identifying Unassigned Products: Go to Products and filter the Supplier drop-down for Unassigned as shown below:
Note: the Supplier field in the list view will appear yellow to highlight unassigned products.
Bulk Assigning Products: Go to Products and click on the Bulk Assign Suppliers button.
When you select a Supplier from the drop-down, the products already assigned to them will appear on the left in the Products section. Select the products on the right-side that you want to assign to this Supplier, then hit the green arrow << button. Conversely, select assigned products on the left-side and hit the red >> button to unassign them.