caffeine in particular is one that affects people differently based on a bunch of different factors.
if you normally drink x amount of caffeine, your body gets used to that amount, so much like with other drugs you can build up a tolerance to it. it doesn't mean you couldn't get the jitters, it just means it would take more to get you to that point.
some people's brains also function differently, and for those people, stimulants can have a more calming effect in the right doses. some stimulants increase dopamine production (nicotine, Adderall), some prevent/delay reuptake of dopamine (methylphenidate), some increase availability of dopamine receptors (caffeine), and each gives the dopamine a chance to work as intended (assuming it was not functioning appropriately before).
if you normally drink x amount of caffeine, your body gets used to that amount, so much like with other drugs you can build up a tolerance to it. it doesn't mean you couldn't get the jitters, it just means it would take more to get you to that point.
some people's brains also function differently, and for those people, stimulants can have a more calming effect in the right doses. some stimulants increase dopamine production (nicotine, Adderall), some prevent/delay reuptake of dopamine (methylphenidate), some increase availability of dopamine receptors (caffeine), and each gives the dopamine a chance to work as intended (assuming it was not functioning appropriately before).