What's the paper quality used for the Penguin Books Great Ideas series?

I’m trying to learn more for a self-publishing project, and I really love this series: https://www.penguinrandomhouse.com/series/BM5/penguin-great-ideas

I’m an amateur so I might not be using the right terms.

Does anyone know exactly what paper material they used for the cover? It has a very weighty, timeless, paper-y feeling; compared to many other books with plastic covers (whether glossy or matte).

I’m asking about the outside cover specifically, but am curious about the pages as well.