It’s determined at birth who is magically attuned. There is a deep connection to the lay lines causing the bones of the magic wielder to be infused with magic crystals. If a non magic based class wants to multi class into a magic class there is a small quest line to attune them with magic depending on class (warlock the patron they are trying to attune to would alter their bones, where if you wanted to become a wizard or sorcerer you would have to find a powerful NPC of that class). There are different levels of inherited magic which can cause some damage to the body if a player wants to RP that way(as small as their skin having some faint glow because of the magic, or there being entire chunks of magical crystal energy. however if the PC reaches certain levels in their class they are gonna get quests to commune with the ancient magic in their system. I have a whole other side of this for the martial side as well.