Magicians are people, who are able to use "magic tools". Not every human can use them, so only those who can are called "magicians".
Many of the magic tools used by C3 are produced by Izuna. Key element of using them seems to be the ability to imaginate what the tool can/should do.
The keys are the easiest form of magic tools. Yumikage, Tsurugi and Junichiro explain their usage like: "Imagine putting it into an invisible keyhole and release the dark powers hidden inside you with this key!" (Chapter 46)
Yumikage uses guns as weapons. Like Izuna explains, they have also created guns, that shoot beams of artificial sunlight - what makes all subclass (exept for Tsubaki's) die.