TLDR; Normal magic comes from God of darkness, magic capacity is finite and indestructible, Silver eyes magic comes from God of light.
Well we have from what I can tell six key pieces of information / examples of magic in Remnant.
1) It was a gift from the God of darkness as stated by the deity itself
2) The God did not take magic back as both Ozpin and Salem still use magic prior to the God's departure.
3) The children of Oz and Salem were capable of using magic, possibly giving it a genetic factor.
4) No people except for the SEW's were capable of using magic after 'the flood'.
5) Magic is tied to the soul as shown by the maidens and Ozma who's soul always retains its magic regardless of his body and the maidens which were stated to have their magic bound to their soul.
6) The silver eyes magic comes from the god of light, unlike normal magic.
Known magic users so far are: Ozpin, Salem, Ruby (I'm counting Silver eyes as magic), Raven, Cinder and Qrow.
Speculative / unexplained properties of magic include - Why it passes from maiden to maiden, to what extent is Ozpin able to transfer his magic and the difference between the silver eyes and the magic of Oz and Salem.
My personal take on it is that capacity for magic is indestructible however can be exchanged and shared between people. This means that parents transfer some (or all) of their magic capacity to their children upon conception or upon death and was originally part of the essence of the God of darkness. A persons magic ability is limited like a battery and the act of using magic is like draining it. However the battery naturally recharges. Ozpin sharing his magic is like lowering his capacity whilst giving the recipients a limited magic capacity.
Alternatively instead of a batter metaphor it may be they simply have a limited magic 'power', meaning amount of magic energy they can use per second and that's is what is shared among maidens.
Possible theory is that Silver eyes are a side effect of the reincarnation process. The god of light used his magic to give Ozma reincarnation however children of him and non magic people receive residual magic from this process. This could explain why his children are seen to have silver eyes (I'm talking about the two children of the guy who looks like a Walmart employee) and why Maria Calvera states the Silver eyes come from the god of light.
Keep in mind this is all speculative.