The signal only needs one conductor. To travel along that conductor (the banana cable) the signal needs a path to earth (or ground, if you prefer.) All you need is for the source and destination of the signal to agree about what earth is. The simplest way to do that is to have an earth terminal on the interior of each module and have all the earth terminals wired together.
If you want to introduce a Kosmo patch signal you have to have an adapter, which can for instance be in the form of a 6.35mm jack that takes the ring to earth and the tip to a banana jack from which the signal can be patched to its destination using a banana cable like the ones I show in the pictures above.
The biggest attraction of this, for me, is that I get to make up my own patch cables for pennies and it takes a couple of minutes (or less) with a wire stripper and a screwdriver.