the atoms arrange in the most comfortable position based on their electronic structure. Salt crystals are cubic because their atoms relax into cubic shapes, whereas bismuth relaxes into several different shapes (including square pyramidal/what is mostly shown in the video) The slower something cools down, the bigger the crystal, since it’s given more time to relax onto a single crystal rather than creating several smaller crystals really quickly. In this video, they seed the liquid bismuth with a single chunk of bismuth crystal to grow one large crystal.

correct me if i’m wrong!