Magical energy is directed through circles. It is pulled into the circle through sources, routed through a number of sigils to transform it, and then pushed out through focuses.
A source or focus may be a contagion (such as blood or a fingernail), or a sympathete such as a drawing or representation of a true name. A contagion forms a more efficient link than a sympathete, and the greater the likeness the more efficient the sympathete.
A spell to launch fire from one's hand might work by using a fingernail as a focus.
Most commonly, a spell is powered sympathetically through a sigil granteb by a supernatural creature in some sort of bargain.
Alternately, you can use blood to power a spell from a willing person through contagion. In practice, it takes a lot of calories to create a fireball, so this is rarely practical. You could also in theory power it sympathetically with a depiction, but the loss of efficiency on top of the difficulty makes this ineffective.
Magic energy is likewise transformed by passing it through sigils.Some sigils are common knowledge. A fire spell might simply pass through an alchemic symol for fire (△) though a less ambiguous sigil might be preferable (火).
These tend to be fairly simple spells. More complex spells generally are granted by a supernatural patron that has an applicable portfolio. They tend to be more specialized, though they might be more powerful or capable of complex tasks. You might protect your home with a generic barrier sigil, but if you were to be granted one by Terminus...
Something more complex like mind magic might be only available through an applicable pact.
Fire granted by Hephaestus will be more tightly focused than a generic spell, while that of Vulcan is likely to be chaotic and might actually form as ectoplasmic lava.
In general, energy humans expend through their emotions winds up in the nevernever. If it was directed toward a particular supernatural creature it tends to wind up with that one.
So, currently worshiped gods have a very healthy income. Older ones seeking more power have to be more opportunistic.