They have used the ordinary looped voice and connected it to a synthesizer using a vocoder. And vocoding is a pretty big subject, so simply Google/YouTube vocoder tutorial. In this case, pay extra attention to the part where they tell you how to connect the vocals and the synthesizer (you can use a vocoder on the vocal mixer track as an effect too, and that's not relevant in this case unless you want to learn it anyway).
When you have connected it properly you can just play it with the synth and tweak stuff using the voice as a timbre.
Though I'm not fully sure how they've tweaked the voice. But it sounds like a greater amount of highpass filtering and then simply playing on lower notes making it pitch down. Maybe a 2-voiced unison with a lower detune too. They have used automations to pitch drop the "synth" and also increase the amplitude of a vibrato-lfo. And then ofcourse some echo.
That's all I can come up with, hope it helps you out.