Magic is a property of reality that the Veil normally suppresses. On the other side of the Veil, in the Shadow, in Domains, in Nod, it is ambient.
The reason mortals on Earth do not encounter magic is that the Veil acts as a filter, damping whatever force or field magic operates through. This is why prolonged exposure to Nod transforms people (they are soaking in an unfiltered environment), why moments of extreme emotion can tear the Veil (the internal state momentarily overcomes the suppression), and why Aeons protrude into reality rather than fully existing in it (the Veil limits how much of them can manifest on Earth).
Eldritch Language
Eldritch language is the mechanism by which practitioners interact with this ambient force. Language here does not mean spoken language alone, but a structured system of symbols, tones, and intent that lets a person shape what the force does. Scribes externalize it in ink. Speakers channel it through voice. Shapers impose it through introspective will. Singers channel it through music. Each discipline uses a different grammar for the same underlying language, and each is difficult enough to master that few can. Most practitioners only learn one.
Talent and Training
The capacity for magic is innate, but the discipline is learned. Some people are born with sensitivity to whatever the Veil suppresses, but raw talent without training leads to involuntary crossings, uncontrolled transformations, and the kind of dangerous accidents that make mundane people afraid of the arcane. Training is what turns a latent Shaper into someone who can reshape a room without killing everyone in it.