I would guess they use an inert substance and actually snort it. Maybe Manitol.
We’ve had actors snort “coke” twice in our films. Neither time did we have anything that’s safe to snort. The first time we used flour and we used angles so that it looked like the actor was snorting. The second time the actor had a stainless steel tube. He makes the motion, then there is a closeup of the tube as the “coke” is drawn in. That wasn’t a macro shot at all. We used a stainless steel vacuum cleaner hose and sucked up powdered sugar. Since we established the steel snorting tube, showed the actor laying out the lines and bending down, and then showed a steel tube sucking up white powder, the illusion worked.