Spells are not granted by classes. So where do they come from?

Well, another, better question is: what are spells a reward for? And what other rewards can one get for the same achievement?

Short version:

• Spells are unlocked by (or rewarded for) skills and made permanent as resources
• Spells pay for their power with Mana

The skills that unlock spells are akin to weapon proficiencies.

The resources that make them more powerful are akin to magic weapons.

0 Mana spells are about as powerful as proficient weapon attacks (or cantrips).

1+ Mana spells can do more, natch.