While watching the video below, I was intrigued, when this interview questions was discussed (timestamp 56:30-57:35).
It made me think about my response, had I ever been asked the same. And it did not take too long before the answer was clear: the B-Tree. There is a very good section on it in Martin Kleppmann‘s book “Designing Data-Intensive Applications“. And I highly recommend this book anyway.
But as a starting point on B-Trees the following video is also quite helpful: