This song was made using only a Volca Sample playing a single sample!
The Volca is playing the same sample every 16 steps, but in each step the sample is pitched to a different note, from C to D+2. What you hear in the beginning of the song is the Volca playing the 16 notes, for 2 bars. I then turned on the "step jump" mode of the Volca Sample, which allows me to jump to one of the 16 steps, effectively playing notes. If I keep the step pressed, the note will keep playing every step (it jumps to itself!), and if I keep multiple steps selected, it will arpeggiate between them.