The track is constructed procedurally, so it's different each time you play it, but remains static after it's been constructed. There are about 20-30 "chunks" that get remixed into the track at random, except for a small number of "black listed" tracks after certain chunks, where they would make the track too hard. Additionally, some of the more difficult chunks only appear after the first 20-30 seconds.
This is a time-tested strategy for simple procedural "endless" games, starting with Adam Saltsman's Canabalt in 2009, and one of this game's inspirations, Terry Cavanagh's Super Hexagon from 2012.