The QBO integration is a 1-way sync from your portal to your QBO account. Once an invoice is in an Open state, a matching invoice and customer (if the customer doesn't yet exist) is created in your QBO account. From there, changes to the invoice and payments are tracked and sent to your QBO account.
Each line item in an invoice is associated with the default Sales service setup in your QuickBooks account.
Note: Since this is a 1-way sync, when you create an invoice in QBO, the invoice is not also automatically created in your portal.