The trick is in delays with high feedback, allowing you to layer lots of single notes or small details over each other, gradually building up a rich texture. The more dynamic range and subtlety you have with your sound source, the 'deeper' you can make the texture. There are a few layers like that in this piece, a couple of examples being a texture masterfully performed by Luke Bozzetto on guitar and delay pedal, and a texture built up of marimba samples played on a keyboard. By using feedback settings on the delay which are slightly less than 'full', you can find a sweet-spot in which the texture you have created so far will decay at a rate matching the new texture you are building as you add more notes, resulting in an organically morphing or shifting texture.