The choir actually sings the solfeggi letter, Do, Re, Mi etc, almost like a meditative etude. The melody is distributed among 4 voices.it is octave displaced, if it were to be played in one octave, it would be too predictable. But by simply displacing the octaves, he gets these wonderful clusters that are fading in and fading out. Soft clusters from intervallic landscapes happen on top of each other, constantly creating harmonies without functions. The end result is a profound beauty from one of the most simple idea, a scale that almost every musician learns on their first lesson. Normally used as a mere warm up or included in a practice routine, only one scale, if thought well can produce a beautiful outcome. Although very simple, one could get various compositional ideas. You could use any scale, in ascending or descending forms, triads, arpeggios, distribute them differently for different orchestrations. If you take a look at the structure, you could see that it does not always start around the same voice and the patterns are well thought and structured. Throughout the entire piece, each voice gets to start the pattern. There is also a dynamic structure to the piece with creates another layer of depth. The constant crescendo creates tension and expectations but due to the never ending harmonic vagueness, it only resolves by decrescendo.
A very similarly structured peace, De Pacem Domine, is also based on the floating and meditative construction of Solfeggio. One main difference is that, it is not based on an ascending scale but it is more based on a floating tonal harmonic movements.
The Alto part having the main M-voice, Soprano section is singing an open position, superior T voice.
The Tenors seem to be altering between different positions of inferior T-voices to Alto part, to create intuitive harmonies.
The Bass section sings parallel notes to Alto section using D Harmonic Minor in 10ths.
These roles stay consistent, working like a perfectly designed mechanism. And displacing the rhythm to different parts creates these lingering harmonies that just shift towards each other.