The arrangement of stars determines what they look like when they burst in midair. A circle-shaped pattern yields a circular firework. Star-shaped patterns give up star-shaped fireworks, and so on.
The timing of what explodes when and with what force can be controlled with precise fuses and thicker wrappings.
Even the noise fireworks make depend on what they're made of. Certain fuels, for example, can make flying fireworks sound like they're whistling.