Click in the upper right corner to get up to speed on all of the new ZBrush 2021 features, including dynamic cloth simulation (although you don't have to limit it to cloth!!), cloth brushes (make your own easily!), new ZModeler functionality like edge extrusion, surface snapping, auto vert welding, topology techniques (remember you can use simulation and collision volumes now!), microPoly for real-time instance mesh replacement for some insane surface effects, animation caching, symmetrical off-axis Nanomesh sculpting, new file formats (like saving a model as an image?!?), new cool macros, and much, much more!!
If you're wondering how those demo videos were made, check out the ZBrush 2021 What's New PLUS Bonus on the marketplace! There's an extra 30 videos covering the "making of" most of those demos - 5 extra hours of instruction, web links to relevant extra online content, source file ZTools on some projects so you can follow along, and even the final render scene ZTools for most projects!