The first 30 seconds of the video explains the best method of adding products and importing inventory levels from multiple channels at the same time. The video then proceeds to show you an overview of the inventory manager.

TIP: You can import inventory quantities from sales channels when you import listings from sales channels. You can do this at any time.

Two other ways to change inventory quantities is to enter quantities directly into the Inventory Manager page or to download an inventory stock list csv file from the inventory manager, adjust it, then import it again to adjust your inventory levels in bulk from a csv file.