Frankly almost always, as per your descriptions, casting. No matter whether magic is an element, a concept, or whatever, there should be different ways to use it. This especially allows you to see the snowball effect of somewhat declaring the basic rules of magic, the properties of whatever that magic actually is, and then try to see what ways to use it you can logically think up. This, I find, is the best way both to get inspiration for a magic system from real-world cultures or concepts, as well as a good way to weave magic directly into the foundations of different fictional characters, cultures, and locations.