Short answer
HubSpot's KB: with one-way sync to HubSpot, HubSpot invoices are created from Xero invoices; one way to Xero creates Xero invoices from HubSpot ones. If you turn on credit card fees, the fee won't sync to Xero, which can need manual reconciliation. Some fields need a custom mapping, which requires Data Hub Starter or higher.
1. Data integration
Data integration in my demo portal.

2. QuickBooks
See QuickBooks Online.