Hello @Adam101,
Just by typing "credit" in the search box of the "Help" window I found the article below.
The first line may explain why you couldn't find where to see the credit details when booking.
The first line in "Things to know" explains why you can't use your credit to extend a reservation.
Your available travel credit will automatically appear on the checkout page. If the travel credits aren't automatically added, it means the reservation doesn’t qualify (trips must be $75 or more).
Things to know about travel credit:
- You can’t apply a travel credit to an existing reservation.
- It expires one year from the issue date.
- Travel credit can only be used once. If you cancel a reservation that had credit applied, the credit can't be reused.
https://www.airbnb.com/help/article/1367