Short answer
HubSpot's KB: survey response properties don't appear on contact records; values are unique to each response, so a contact's later answer doesn't overwrite an earlier one. Use them in segments, workflow enrollment triggers, and single-object or cross-object reports. You can create your own too.
1. Surveys
Feedback surveys in my demo portal.

2. NPS rating
The FAQ notes NPS answers also land in the Last NPS survey rating property.