An ice crystal grows from a seed, preserving the hexagonal symmetry of water's lattice. Branch tips stick out into air with more access to vapor, so they grow faster than the hollows — that's how a branching dendrite forms. Per the Nakaya diagram, the shape of a real snowflake (plate, column, needle, or star) depends specifically on the combination of temperature and vapor supersaturation in the cloud; here it's simplified to two sliders.