Not being a religious person I believe in the basic return to the nothingness of pre-conception. I don't really think about it much any more, but I have a particular memory of thinking about it strongly and in a brief moment I was able to grasp the concept of non-existence; nothingness; just the black. It was an odd, profound, memorable moment that I haven't been able to re-create (though haven't particularly tried). Thanks for the reminder. I think that moment possibly aligns with a meditational state of mind. Now I'm going to have to try it again.
I'm (still) reading the Mars Trilogy by Kim Stanley Robinson in which gerontological (sp?) treatments can effectively reinvigorate DNA to keep the brain and body from degrading for hundreds of years. There are some interesting active and passive explorations of the implications of that within the books. There was a particularly great sentence about memory, and when you get old you essentially get lost in the memory of the various experiences you had in a particular place, and you can't really control how those memories are triggered and 'come flooding back'; you're eaten up by your own history, and the longer that history is the less able you are to see what 'is' as opposed to what 'was'. It was a beautifully written succinct sentence.
Also have recently read and watched Altered Carbon. The entire concept-base of the novel (and therefore the shows) is the ability for humans to live forever and investigating some of the extended consequences.
Both the Mars Trilogy and Altered Carbon have varying amounts of focus on the accessibility of eternal life only to those who have the means. That should seriously scare anyone that's not in the 1% of the 1%.
Another book I've read, Silver Screen by Justina Robson, has a character that digitizes their consciousness. This manifests as having a certain power in their ability to have their consciousness traverse interconnected data networks, but the friends of this character can tell something is "off". It mentions that this digital consciousness can no longer 'learn' as a person; they are stuck as the personality they were at digitization.
Technologically, I think we're a long way off digitizing our consciousness. It takes months to find a vaccine for coronavirus. How much can we possibly actually know about the function of the brain and mapping it and expecting to be able to re-create precisely a person consciousness within electronics? What psychological disorders will we create along the way? (I think Ghost in the Shell SAC dealt with a thing called cyberbrain sclerosis, Johnny Mnemonic had Nerve Attenuation Syndrome).
I consider 'what happens after we die' to be a solved problem. The ability to extend life, and the resulting implications, there lie science fiction treasures.