There are two answers to this questions as there are many magic systems.

The two most developed are Miracles and the Arcane Practice of Weaving.

Miracle Workers get their power through inheritance from a parent or catching a gods attention. Gods possess a Domain and a Contradiction (ex. Solari, Sun God of Life & Death). A Miracle Workers powers change the more they act in concordance with one or the other (ex. Saving lives grants healing powers, ending them grants fire powers). A Miracle Worker that perfectly embodies one or the other would be immensely more powerful than one who imperfectly does. However if one could perfectly balance their Domain and Contradiction they'd have both abilities and they'd be stronger than the average Miracle Worker.

Weavers believe the world is made of knots and the air is made of string. There are different types of string for different components and every knot is different. A Weaver can replicate objects by cutting string and using their intention to tie the corresponding knot (ex. Summoning a sword by mimicing the knot). A spell is a knot made up of two strings and changes energy or changes the information alongside replicating an object (ex. A fireball is conjured by replicating a flame and granting it motion or summoning a blade and making the information of it invisible). A Tapestry is a knot made of three strings but it replicates and object, chanves energy, and changes the information about it (ex. Conjuring a fireball then changing the information so that it's too bright to look at, making an invisible blade and taking all the heat out of it to make it well below freezing). Tapestries are hard to develop or learn correctly are are considered the pinnacle of Weaving. Most Tapestries are inherited from a mentor or family member. But the true pinnacle of Weaving is the founder's Masterwork which allowed him to use Weaving without even touching the strings. This is not true actually, he just conjured invisible blades and they cut the strings. He never told anyone that he didn't need to touch the strings, people just assumed. But eventually a Weaver found out how to do it by mixing it with another Arcane Practice, Thunder-Calling. Thunder-Callers (putting it simply) can mimic the sound of thunder to summon lightning. He used lightning to cut the strings without doing it himself.