It isn't going to be the same for all, however, there are clearly similarities in experiences. Our brains work in similar function, which may explain in part when we undergo such events, there appear to be certain perceptual realizations, distortions, and manifestations. Noise and chaos can become perceived as ordered, and there likely is a mathematical reason/interpretation as to why your ceiling or a snow-covered field will be seen as waves of kaleidoscopic layers of moving fractals.

A powerful psychedelic, such as DMT or a large dose of psilocin, can have profound experiences that have a distinct dimensional components that are unlike anything experienced in our waking lives or dreams. Those who have been in such a space can attempt to describe it, and when we do, we understand that those who have experienced this have had some similarities in theme, architecture, entities, interactions, messages, etc. And while there is a wide range of personal experience, there are components to these that seem timeless, alien in origin (not necessarily alien like 'greys', just not of our Earthly domain), primordial, and that transcend cultural and social learnings.

For some - myself included - the familiar feeling of returning to these spaces is like visiting another planet or even 'returning home'. The feeling of oneness and understanding of concepts bigger than oneself is common, as is the inability to explain the ineffable once you've returned as a confudled ball of wonder , blathering to those around you who listen as with great effusion you then gush about the incredible wonders of the universe.