Each type of spell caster forms magic into reality based on their understanding or affinity for the rules that govern the type of spells they can cast. Below is a list of the various spell casters and a brief explanation of how they gain access to the invisible forces of magic in the universe. Typically, this is done through tapping into the various planes of existence outside of the Material Plane.
Bards are unique in that they have the mental flexibility to learn the rules of magic from any source. This is still limited though and their primary list of spells is mostly based on manipulation and beguilement. Anyone familiar with the intrigue of the Courts in the Fae Realms will recognize bards as employing the same set of rules to form the majority of their spells.
Clerics and Paladins are granted bonuses to their spell casting abilities through worship of various Deities. However, their practice of the divine arts is tied almost entirely to the rules of magic in the Astral Plane. The rare exception to this are those who interweaves rules from other planes, with the blessing of their god, to cast spells that are less than benevolent.
Druids and Rangers tap into the Atral plane for some of their abilities but are unique among casters in their deep understanding of the natural world here on our Material Plane. With this they are able to bend to rules of nature to their will and even communicate with natural beasts and plants. Druids take this a step farther and can even assume the forms of various animals. Some study the Primordial for a deeper understanding of elemental magic and can even turn into elementals for a short period of time.
Both sorcerers and warlocks gain their magical abilities from the Primordial. Sorcerers are able to call on the raw form of magic as easily as you or I breathe. It’s intrinsically part of them. This natural aptitude generally means they aren’t as prodigious as a Wizard, but the spells they are good at, they can warp and weave in ways even the greatest wizard can’t match.
Warlocks make a pact with the Grael. Sometimes unintentionally, sometimes on purpose. This pact affords them all sorts of abilities uncommon to your average spell caster depenent upon the whims of the Grael itself. After their pact is made, their magic is inherently understood, like a sorcerer, but with whatever conditions the Grael set. Warlocks are more likely than any other type of magic user to gain supernatural abilities from their use of magic.
Wizards gain their knowledge through study of arcane lore and rituals. Their affinity for destructive magic means the majority of their ruleset comes from the Infernal Coils. For them, it’s always a balance between containing the forces at their disposal and destroying everything around them. Wizards specialize in various schools of magic, some of which require study of magical practices in the other planes. Their base knowledge still comes from the Infernal Coils however and any knowledge gained elsewhere is just used to “adjust” the rules to fit their particular school.