Harnessing Magic

Mages tap into sources of power greater than themselves to channel and shape that power into the effects they want. It’s a halfway between a D&D Wizard and a D&D Warlock without the spell slots. A “power source” can be an elemental plane, a former master mage, a God, some other kind of innately magical being like a demon, angel, or dragon, etc. Unless their patron forbids it, they can generally “tap into” multiple such sources. This is why most canon spellcasters who do not provide their own power have spells referencing multiple such sources. The power source itself doesn’t even need to be magic. For instance, the Schattenwelt is used for powering both Shadow Magic (Thule Society, aka Nazis) and the Black Bands of the Blackguards (Evil Green Lanterns from Deep Space), and only the former is “magic”, because the difference between, say, magic and cosmic power is how the energy is tapped into and processed into creating its effect.

The Worlds of Magic

There are your typical things that you find in many fantasy settings: Astral and Ethereal Planes, Seelie and Unseelie Courts for the Fey, etc. Accessing these places is another matter entirely though. Things like portals between dimensions are very difficult to make without proper tools like the Alternity Atlas. In our world, magic used to be a common thing most people could do simply because it filled the air and was easy to access and manipulate (similar to D&D). Unfortunately, it no longer permeates the world so thoroughly and is nearly impossible to access. This is why magic is now “drawn down” from these rare power sources. Mages who don’t do this tend to be the extremely rare exceptions that innately have access to magic by their own right - such as Medea, or the reigning Master Mage (of which there is none).