This is a continuation of my Specifics of magical symbols and casting series of questions.
In the process of splitting my post up, I noticed my final question got a bit broad. So to specify how magic works in this world:
Magic in this world is omnipresent, yet largely like science in discovery process. It holds no consciousness or purpose, but acts like a program, "seeking" similar patterns. Runes are largely created in the "library" of magic by association.
For instance, a drawn flame in a cave to signify fire, used again and again over countless generations, created the association that this symbol means fire in the "Magical Library" that permeates all existence. In our world, this relates to a Deep Learning AI, except... there's no code, no point, no purpose. It simply, is.
This also opens up the possibility that, either in densely magical areas, gathering of like minds, basically permeating, society-wide philosophies can create deities, manifest purely-magic beings, or infect a town with fervor, inexplicably turning everyone within the region to the same fervor if they aren't strong of will. Ultimately, this component of nature that just is actually could gain sentience, however, due to the nature of the beast, it could never truly claim autonomy over the entirety of magic.
Magic exists regardless of belief in it - however, belief or mass fervor can and often will generate a certain outcome. Say, people feeling hopeless in a ravaged, pillaged, swamp town may always be surrounded by darkness. Sudden, widespread happiness won't make this disappear in an instant, as it is a magical abstract that has built up over the region that will only go away either in time, or if diminished and replaced by a fervor of emotion that would combine or overwrite it.
One caveat, however, is that you couldn't just get a bunch of random people together and say 'draw this over and over again,' as new functions of this "Magic Library" must draw from some sort of mass phenomena, where some emotion is in a state of mass hysteria among a region, or a belief isn't just skin-deep. The stronger and/or more universal a belief and/or symbol is, the wider the scope this magic can be reliably used in.
With this known, what would make it difficult to master the physical concept of manipulating magic?
For instance, a character interacting with something they don't necessarily fully comprehend, but 'intuitively' (or through past experience) understanding how to use a magic function or manipulate a new (relative to the individual) substance?