How does caffeine exist?

Caffeine is a natural chemical found in some plants like coffee and tea. It doesn't "get into" them at all - it comes from them. The plants manufacture the chemical as part of their natural growth. It was originally part of the plant's defense, because it makes you feel wired and jittery and most animals don't like that feeling - but humans do so they started growing the plants just to make coffee and tea out of them!