My thought was that they might have been from the Daemons/Daemonesses Themselves, which They designed, but then I realised that the Sigils are geometric-like. That led me onto the thought that the Sigils are not by design, "Ooh, I fancy this. That looks nice. No, that bit looks crappy so I'll remove that. There - a Sigil fit for a Daemon/Daemoness." More like they are Naturally-inspired drawings, with, e.g. the lines in Runes corresponding to particular parts of the Soul, the geometry in the Sigils correspond to the Brain/Mind, which the Human Brain/neurons look like the Universe, which seems to fit together.
Maybe ignore this next part -
(I speculate)The geometry of the Sigils are of, to, for, regarding, about the very Nature of the Daemon/Daemoness and His/Her specialities or Natural abilities. They, being so powerful, can perform any act, as far as They are advanced and as long as it is possible Physically, but Their Sigils are more specific for things, meanings, intentions.