So I understand how saving throws "work" in general as in what you test against.
However some spells just call for a "constitution" check for example for saving throw.
for example Poison Spray:
You extend your hand toward a creature you can see within range and project a puff of noxious gas from your palm. The creature must succeed on a Constitution saving throw or take 1d12 poison damage.
So the monster will use their constitution modifier with the saving throw...but whats the requirement...like how do we determine what number it has to be above? Poison spray doesn't say it needs to be above 12 to succeed the saving throw for instance?