I knew a few steps that'll help record your purchases in QuickBooks Online (QBO).
You can either set an initial/beginning quantity and value when setting up your products and services or manually record the purchases in QuickBooks. Let me walk you through the process:
To set up the item with initial quantity and value:
For further details about setting up products under the Products and Services, you can check this article: How to create and use a products and services list.
On the other hand, you can enter/record a bill or an expense if you wish to record the purchases for your inventory and supplies manually.
For additional reference, you can check these articles:
The steps provided should help record your transactions accordingly. If you have further questions about the steps provided, just add a comment below. I'll always be here to help you. Have a great day ahead!