You have the option to manually add inventory for a launch on the portal, but there are also two convenient import methods: CSV upload & Shopify import. This document explains how each method works and how they differ, which should help you choose which one is appropriate for your situation.
Examples:
To import inventory and variant details, go to the Launch Creation form in the Portal.
For CSV Upload, scroll down to the inventory table and select Imports.
To import Shopify product information, scroll down to the Shopify details section.