Purchase orders are how restocking gets tracked end-to-end - from what you ordered, to what actually showed up.
Go to Purchasing → Purchase Orders and click to create a new order. Pick the Supplier, an order date, and an expected delivery date.
Use Add Item to list each product and quantity you're ordering. The subtotal, tax, and total update automatically.
Submit it - suppliers linked to your account as companies can see and respond to it directly; others you'd simply order from offline using this as your record.
When the delivery arrives, open the purchase order and use Goods Receiving to check items in against what was ordered - this is what updates your inventory counts.