there's a few synthesis libs they could be using: max, ChucK, SuperCollider, Impromptu, CSound
there've been some relevant threads e.g. https://news.ycombinator.com/item?id=7571082
https://news.ycombinator.com/item?id=8344827
(i've never tried any, still geting to grips with engines like DSI mopho, korg ms2k, blofeld, minibrute, bass station etc