It's "regular old" FR-4 PCB, so far tested for up to 8 layers. Industrially copper inkjet printed, then drilled and plated, completely forgoing wet etching processes. Only through hole vias are supported[1] at this moment(but that's good enough for a lot of use cases).
1: https://twitter.com/shinyashimizu_j/status/18688838600435673...