How to add products to supplier product list manually

Go to Manage Suppliers → Click on one Supplier line → The details page of that supplier will be shown.

Here there is a Product section with 3 ways to add products.

To select products manually, click on Add products button

The popup of available products in your Shopify store will be opened. You can add any of them to the current supplier.

Next, click on Add to finish adding products to this supplier details.