Q1. What is the difference between invoice notes and internal notes?
Invoice notes are visible on the invoice PDF.
Internal notes are only visible to your internal team in the invoice settings.
Q2. Why has the invoice notes section been automatically filled in?
When creating an invoice, if you select a contact that has default invoice notes filled in, those will automatically be applied to the new invoice.
For more information about default invoice notes for contacts, refer to What is the "invoice notes" section for a contact?