I run pastry making biz. How do I use QB to tie my sales (finished pastry product) to my raw materials inventory such that each sales will reduce the inventory automatic?

Thank you for raising you concern here in the Community space, @juicychopsng-gma. I can provide you with some information that will help you resolve your concern.

You can only set up and track inventory items in QuickBooks Online (QBO).The option to provide inventory assembly is currently unavailable.

I'd suggest using a third-party app that integrates with QuickBooks since QBO's metric is by quantity, and ml or liters are unavailable. You can check it out here in our Intuit QuickBooks Apps and follow the steps provided below.
 

1. Login to your QuickBooks Online account.
2. Go to the Apps menu.
3. Type in the clue word of the specific app or the one that relates to the task you're looking for.
4.  Click the magnifying glass to search.
    

You can check out this article if you need additional information about inventory management in QBO: Quick-Look FAQ: Inventory Management.

You can run a report in case you want to take a look at the things you buy and sell, and the status of your inventory. Check out this article for more information on which report to use: Use reports to see your sales and inventory status.

If you need any further assistance managing your inventory, ping me a reply and I'll surely get back to you. Have a wonderful day!

@juicychopsng-gma 

QBO doesn't offer the "assemble" feature to track the raw materials and end products separately. You need to have an additional app to integrate with your QBO. Otherwise, consider migrating to QB Desktop Premier.