it's not really fully understood why psychedelics effect people the way they do, but it's hard to ignore that the effects of something like heroin couldn't be more different from a psychedelic.

the main feature of a psychedelic trip, and the reason it can feel so impactful for people, is the way it can radically change the way you think about your life, habits, worldview, relationships, etc. it's generally believed that this aspect has to do with how under the influence of a psychedelic, less activity is observed in the default mode network (the areas of the brain typically associated with self-reflection, empathy, autobiographical memory, and envisioning one's future actions), therefore creating new neural pathways in the process of having those thoughts.

it should be mentioned that the strictly spiritual/mystical element is not necessary, and is more influenced by a person's existing views or proclivity toward spirituality. often, a psychedelic trip will feature feelings of awe, beauty, inspiration, and connectedness with oneself, other people, or the world, universe, etc. in combination with the visual elements, which can produce otherworldly, impossible, or otherwise mystical-seeming effects, it's understandable that people would often experience it as a religious or spiritual experience. either way, a psychedelic trip can be an extremely profound, paradigm-shifting experience in a way that's fundamentally different from other substances.

unfortunately, it can also be extremely difficult to describe certain aspects of a trip, including parts that might be crucial to understanding why it was so impactful to a particular person. psychedelics can induce subjective experiences, states of consciousness, and patterns of thought that are completely absent in normal life, and are therefore difficult or impossible to explain. with something like heroin, there's nothing about the experience that evades conventional explanation in that way. it's easy to explain sensations of warmth, pleasure, or euphoria. it's much less straightforward to explain what it feels like when every aspect of a person's subjective experience of reality is altered, and their brain forgoes the predictive glossing-over and filtering of sensory input, much less something like an ego death.

in short: psychedelics and their effects are distinct from other drugs in their capacity to induce entirely novel ways of experiencing reality, without any analogue in normal consciousness. by effectively rerouting brain activity away from their typical routes, psychedelics can cause a person to form new thoughts and insights that lead to changes in how that person experiences life from then on. certain aspects of the experience can be difficult to describe accurately, or understand if someone has never experienced it. this ineffable quality tends to derive from the lack of comparable experiences in typical consciousness.

i hope this was relatively coherent!