What are the beads this guy is scooping out in this video: https://www.youtube.com/embed/3Gjd5DRyeks
That look like Hydroton, which explained above is expanded clay.
It is highly porous and holds moisture very well, and is used in a lot of Deep Water Culture/Aquaponic/Hydroponic applications.
These are called by various names but the one I am familiar with is LECA. It stands for Lightweight Expanded Clay aggregate and is made by this company.
Having seen this material used in twenty and thirty year old installations I can attest to its durability. Lightweight? Not so much.