When it comes to magic, I think D&D is a good reference for headcanoning an explanation for how it tends to work in WoW. Until Blizzard comes up with their own explanations, which they likely wont.
There are 5ish main avenues to obtaining magical power. Study, drawing from a source of power, raw faith and willpower, creating a pact to a patron or deity, spiritual transcendence.
Arcane Study: Though the casting of a typical spell requires merely the utterance of a few strange words, fleeting gestures, and sometimes a pinch or clump of exotic materials, these surface components barely hint at the expertise attained after years of apprenticeship and countless hours of study. Example: Jaina, Antonidas, The Kirin Tor.
Fount of Power: No use for the spellbooks and ancient tomes of magic lore that others rely on, nor do they rely on a patron to grant their spells. By learning to harness and channel their own inborn magic, they can discover new and staggering ways to unleash that power. Example: Elisande, Liadrin, Elves who rely on a well of power.
Faith: Harnessing divine magic doesn’t rely on study or training. A cleric might learn formulaic prayers and ancient rites, but the ability to cast spells relies on devotion and an intuitive sense of a deity’s wishes. Example: Tyrande, Anduin, Priests and Paladins
Patron: Through pacts made with mysterious beings of supernatural power, one can unlock magical effects both subtle and spectacular. Drawing on the ancient knowledge of such beings, one can piece together arcane secrets to bolster their own power. Example: Gul’dan, Rastakhan, Cho’gal.
Nature: Druids revere nature above all, gaining their spells and other magical powers either from the force of nature itself or from a nature deity. Many druids pursue a mystic spirituality of transcendent union with nature rather than devotion to a divine entity, while others serve gods of wild nature, animals, or elemental forces. Example: Malfurion, Thrall, Druids and Shaman.