An internal memo seen by the site indicates that the iPhone XR will be available as a loaner starting November 4. Currently, Apple uses the iPhone 8 as its loaner phone.
The Loaner Phone is offered to you while your Customer Phone listed on the Genius Bar Work Authorization is serviced by Apple. Apple will return the Customer Phone to you after service has been completed, the Loaner Device has been returned and all outstanding payments are settled.
Once Apple notifies you that your phone has been repaired you have fourteen days to return the loaner device.
More details in the iPhone Loan Agreement linked below...