This is the behind-the-scenes clip for the Chladni plate segment of Nigel Stanford's CYMATICS video, published the same day. It runs under two minutes and shows the plate mounted on a speaker with sand poured over it, then driven at different frequencies.
The description gives the physics in three sentences: the plate resonates at particular frequencies, divides into regions vibrating in opposite directions, and the sand collects along the nodal lines where nothing moves. The shape depends on the frequency. That is the whole principle, and it is stated more clearly here than in most longer explainers.
For anyone building a plate at home this is the clip to watch first, because it shows the mount, the plate size and the audio path in a few seconds rather than hiding them behind editing.