The envelope generator is a set of controls to make the beginning, middle and end of the buzz tone slow, sustained, and, like pressing the loud pedal on a piano, sustained. There are numerous other voltage controlled routes for the raw buzz - a filter resonance, which adds feedback into the upper partials (partials are parts of a tone above the fundamental note we hear), filter resonance can extremely emphasise the upper tonalities even to a screaming howl. It's all in the subtle tweaking of these voltage controls.
Another voltage control is a "white noise" maker which on a staccatto note sounds like a percussion instrument, a high-hat. The bass line was easy, with the creamy deep Moog voice at about 2/3 filter, so a little upper growl but essentially a delicious bass tone.
The track "Brent" (I'd seen Brent geese over The River Deben the day I finished the track) used these voltage controls to turn a single square wave buzz into all the instruments merely by subtracting parts of that buzz to leave a synthesis sounding similar to conventional band instruments. And at the same time the synthesized sounds have equal quality, as instruments, as more conventional band instruments.