Sequencers, synthesizers, and some classic dark techno sounds. Their albums always sounds like you should be in a planetarium seeing a simulation on stellar formations or something.

YouTube: https://youtu.be/2dE7w046lIA

Apple: https://music.apple.com/ca/album/halo-remastered/351941181

Review: https://www.starsend.org/HaloSiren.html

Halo is a work hovering at the eternal dusk of dreams and interstellar space. It is a musical experience that offers the sensation of passing through something; perhaps a beam from some far-off galaxy or a shower of cosmic debris. The album is intelligently designed and the wonderful transistions between movements draw the listener along easily. Relying on moods created through texture and atmosphere, Halo also offers a strong melodic content which is matched by extraordinary ever-evolving sequencer cycling. The tight analogue tones dance up and down the octave, in and out of synchronization with a throbbing bass rhythm and create seemingly endless permutations of pulse and flow. Above this torrent floats misty consonant choirs cut by reed thin themes traversing the scales.