In Desktop, I create a PO to our supplier, from the estimate to our customer - can I do this in QBO?
It's awesome that you've been using QuickBooks Desktop and it's a great idea to learn if features you're used to translate from one product to the other. QuickBooks Online is a much more streamlined version of the program. The good news is it also has the capability to take information from an estimate and create a purchase order.
When you've created an estimate and save it, there'll be a button beside the total amount in the upper right-hand corner that you can click to either Create invoice or use the drop down to Copy to purchase order. This second option will open the purchase order window with the item already filled out. All you'd need to do is choose the supplier if you don't have a preferred supplier for that product set up.
Here are a few articles about the purchase order feature, which is available in QuickBooks Online Plus level accounts, and estimates.