Not exactly sure how they made it, but this sounds like “generative ambient” to me.
People make music that sounds similar to this using something called a “modular synthesizer” - its basically a big rack of individual synthesizers that each do one thing, and people plug them together to create “patches”. Then, they run a wave generator through them to generate ambient music that doesn’t actually require anyone to “play” anything during the piece.
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Yep. This is one of those posts that should have just been a web search instead.