Demos are almost always generated and displayed at runtime. The music is composed of samples generated at runtime by additive synthesis which is just adding wav

Demos are almost always generated and displayed at runtime. The music is composed of samples generated at runtime by additive synthesis which is just adding waveforms of differing frequencies together when you get down to it. It doesn't take many lines of code to produce CD quality instruments that can be arranged as a soundtrack. Today's 64k intros offload their drawing onto the GPU using shaders and a technique called raymarching which is a form of fixed-step raytracing that can be done in real time. Essentially the objects you see are painted with math: instead of thousands of vertices for a 3d fractal or a complex object, a formula is fed in to the ray marcher and it returns a shape with colors and lighting. Combining shapes using unions and intersections is then done and often the shape formula will take time as a parameter as well. To play with this concept yourself go to https://www.shadertoy.com/browse https://www.shadertoy.com/browse and see how shader programs are rendered into incredible scenes with astonishingly few lines of code. For a wonderful look at the creation process, see the grandfather of raymarching Inigo Quilez create a selfie of a girl from scratch using formulas here: https://www.youtube.com/watch?v=8--5LwHRhjk https://www.youtube.com/watch?v=8--5LwHRhjk