What are the sources of slavery in Islam?

I’m confused as to who qualifies to be enslaved. People often say captives of war, but what exactly does that mean? The opposite forces? Civilians in the town raided? How would you differentiate between a captive and non-combatant? Also, when does the sultan enforce the jizya for the non-believer?, to my understanding Muslims cannot attack the ones who pay jizya, so let’s say the mujahideen attack a place, do they just keep collecting captives till the sultan tells them to stop?