I believe that NDEs are caused by DMT the sprit molecule, which is a great book you should read to follow this up. DMT is an endogenous, highly potent psychedelic which exists in all humans. It is theorized to be the compound which allows dreaming and perhaps consciousness itself. When someone is dying it is not a stretch to imagine that their brain releases all of it's DMT stores since they would no longer be needed and can relieve the agony of living through your own death. In my personal experience as well I can see all of the stories of NDEs and seeing heaven and all of that as being analogous to a DMT breakthrough. I feel like I saw everything, and the last fleeting thought I can remember having before being swept away by the enormity of the universe was that consciousness is not just a mechanical or chemical reaction. And this is coming from someone who's studied physics their whole life and at the undergraduate level. Consciousness is something that is external and shaped by biological functions and the structure of our brains and bodies. The afterlife is nothing more than the borrowed spiritual energy returning back to the collective unconscious.