I used to use DXM quite a bit, which is OTC cough medicine that at higher doses turns into a dissociative. I do not recommend using it at all. But I can detail what it felt like.
In the beginning, pure euphoria. It has similar properties to MDMA as they both work on the serotonergic system. Very euphoric, music sounded amazing like you're living inside of it. You feel like you're a part of everything around you, but at the same time you feel like nothing. Like a pair of floating eyeballs.
You look at yourself in the mirror and it fascinates you because you don't think it's you. You feel like you understand absolutely everything and the meaning behind everything. Sometimes you feel like you can communicate telepathically or as though it's actually happening (this is common among dissociatives). It feels like you understand everything. Your purpose, your existence, other peoples existence, the world it suddenly all makes sense.
Put simply, it feels as though everything around you is communicating with your soul.
It sounds amazing but I do not recommend using DXM or any dissociatives as they are very harmful for you. Nothing is worth your health and I wish I had never touched the stuff.
Stay safe OP.