The servers serve to relay the info and help in discovery between peers i believe. Seth for Privacy has a good podcast he did with Evgeny, creator, back in Feb i believe.

Data is stored on your device from what i understand ( and the receivers device )

The incredible thing about simplex is that you can run your own server, so between your device and your server being run by you...i cant think of any protocol that provides more control over your actual content you senf, as well as your meta data. Its pretty impressive fro mhat ive learned.

With Start9 OS you can run a server with no technical chops whatsoever