When you use PSA, there is an new option set field called 'Work Type' on Leads, Opportunities, Quotes and Orders. This field will be either 'Work-based' or 'Item-based'. You may need to make the field visible on your forms.
When you qualify a Lead that is Item-based, the Opportunity will also be Item-based and D365 will automatically navigate you to the 'normal' Information form. When you qualify a Lead that is Work-based, the Opportunity will also be Work-based and therefore D365 will automatically navigate you to the Project Information form so you can add Project-based Lines as well as Product-based Lines.
You can update the value at any time, although it's recommended to set it as far up the sales chain as possible (i.e. Lead or Opportunity, rather than Quote or Order).