Can't all sounds be "composed entirely out of sine waves"? Isn't that what the Fourier transform is all about?
Good questions.
In a nutshell... Yes. As a general rule, all sounds can be composed of sine waves. The fourier transform breaks down a waveform into a series of sinusoids, each with its own frequency, phase and amplitude. The sum of these sinusoids equals the original waveform. I'm guessing you already know this.
Although I am using sine waves, the difference here is that there is no original waveform. Every bleep you hear is constructed entirely from scratch using sine waves. This is called additive synthesis in the world of sound design. Breaking down vs. Building up.
As for the reverb... After the bleeps are generated, they are sent through the reverb instrument to spatialize the sound. Hope I've answered your questions.