The way this is traditionally done is by writing a demomaker tool, then writing a player for the procedures (instructions) which the demomaker generates.
One can then generate the instructions to the 3D rendering engine and the highly custom software synthesizer in the player. This information easily fits in (less than) 64 KB. The achievement is not in the final result, but in writing the demomaker.
The German group Farbrausch (former Sanity on the Commodore Amiga) was the first to invent this procedural generation technique. Almost two decades ago, they wrote their own demomaker called .werkzeug (.tool).