Currently, creating an inventory assembly in QuickBooks Online (QBO) is not available. However, you can use Bundles as a workaround. A bundle is a collection of products and services you sell together. You'd use it if you don't need to transfer and track the finished products before it is sold. You can check this article for more information: Product and Service Types.
Also, you can find some third-party applications for creating assembly and item group transactions through the Apps menu in QBO. You can also visit this link: https://apps.intuit.com/app/apps/home.
Go to the Apps menu.
Type the app that you're looking for on the search bar.
Click the magnifying glass to search.
I'd also encourage checking our QuickBooks Blog from time to time to stay current with the latest feature available in QBO.