This article explains how you can order items from within project, if you want to order across multiple projects or have a process where employees request products ordered, you may want to check out ordering across multiple projects.
Order Selected Products for a Project
Within the Project > BOM, you can select products you want to order and select the "Add to PO" option to add them to a new or an existing PO for the same supplier in "Draft" state.
To Start check/select the products you want to order
The product sidebar will open on the right side.
Click the split button in the sidebar in the ordering section
Click the Add to PO option