I wish there was more info about how they're made but I suppose part of the demo scene has always been secretive and reverse engineer it if you want to know.

The first one said made with Godot and blender so is it using premade assets? (Which I thought was not allowed)

Edit: #22 Heart of Glass says 144kB of WebGL + audio. I assume this means the audio is separate.

This 64kb intro has speech synthesis to generate the voice of the song.

https://www.pouet.net/prod.php?which=9424

More info: https://www.flipcode.com/archives/08-04-2003.shtml