Creator here. In general, the samples are automatically selected using a mix of onset detection and audio analysis, giving preference to clips that have a clear musical pitch. I give a brief explanation here: https://citizen-dj.labs.loc.gov/about/#how-do-you-selectcrea... . The sequencer randomly selects 4 of those samples that are sequential in the source audio. You can change them individually by clicking on the track settings button (the mixer icon).