It’s sand with synthetic fibers according to the FEI documentation
In Europe, they often drag and roll the footing, so it’s super even everywhere. But most importantly, they really water it. An FEI rider I rode with said it should be wet when you walk the course. They have microphones at the jumps so the sound of takeoff and landing is magnified. But this is as close to wet sand on the beach as you get, which is supposedly the most ideal surface for jumping in terms of grip/cushion. They need both at this level.