Yeah, it seemed pretty clear on listening (to me at least) that they have a trio in front, with a choir backing them. I only hear one point where they all sing together, when they all sing “open, open” near the end. And I’m not entirely sure about the first “open.”
As for the effect of the actual choir, I agree that the soprano dominant mix contributes a lot to the sound. When we think of a heavenly choir, we tend to think of angels, which popular culture portrays as mostly female. The use of constant vibrato is associate with a more formal and older sound, which also fits the angelic idea.
But I’d also add the reverb. It sounds bigger, like it fills the skies. But it’s still in the background compared to the trio who sound closer (by being as loud as the whole choir), which adds some feeling of distance. And there’s the way they sound like an instrument than vocals, singing vowels, sliding up and down to the notes, and varying the vowels. At certain points, I actually thought they were strings for a bit. They blend very well with the orchestra. That way, despite being in the background most of the time, they still sound huge, like heavenly hosts.
What I found most interesting was the point of contrast. There’s a point in the middle where they sing in unison, with a perfectly matched but overly shaky vibrato. It adds this spooky sound to contrast with the heavenly sound the OP mentions. And, checking a clip from the actual film, it seems it is what you’d think: it shows the Wicked Witch on screen.
That contrast also contributes to our perception, I suspect.
(video cued up at the spookier part. Go back about 30 seconds to hear the beginning part. It’s as the party leaves the poppy field towards the Emerald City).