So NaD, but my limited understanding is that psychotropic drugs and psychedelics rely on already in place neurochemical pathways, and excite the shit out of them by imitating naturally occurring neurotransmitters to produce serotonin, dopamine, and a few others.
So basically, the stuff that makes you high and hallucinate is already in the brain, just not in the same amount as when you are high.
Plus, your brain is trained to filter and layer the world into usable information when the levels are normal. When they get higher, your brain no longer interprets the information the same way. This is also one of the reasons why psychedelics are starting to prove useful for things like neurogenesis and addiction treatment. It forces the brain to kind of slightly rewire itself, for lack of a better term.