Magic is its own thing.
According to the D&D 5th edition Player's Handbook, p. 201 (Ch. 10, "Spellcasting"):
Magic permeates the worlds of D&D and most often appears in the form of a spell. [...] A spell is a discrete magical effect, a single shaping of the magical energies that suffuse the multiverse into a specific, limited expression. In casting a spell, a character carefully plucks at the invisible strands of raw magic suffusing the world [...]
- Magical is not physical, per se, although it can be used to create
physical effects, it's magical energies that suffuse the
- It's not "astral", although some spells do affect the Astral Plane.
Dungeon Master's Guide p. 46-48 describes the Astral Plane, and it
isn't described as the source of magic, or made of magic, at any
point. The DM is, of course, free to invent their own campaign
setting where this is the case.