There are multiple designs that can be stitched from the linked circles sashiko pre printed fabric by stitching some lines and washing the rest away...

Pre printed sashiko fabric refers to fabrics that have been printed with wash away sashiko designs so that you can skip the transfer part, and go straight to sashiko stitching.

Wonder about the fabric sashiko was originally stitched on,
and how the designs were transferred to the cloth?

Never done sashiko stitching? Start with this quick how to. Sashiko is very easy to learn, and rather addictive once you start :-)