Have spent considerable time wondering about death and what is actually is. Always thought I could been a good doctor, or you know, mad scientist.
A dead living organism no longer works to sustain its' life. Something stopped working right and it died. But one moment it was alive, the next moment, perhaps a split second later it was dead. But everything's still there. All the atoms and particles making that colleciton of matter alive remains after death. As it decomposes these detach and literally float away to become part of other things. They say we all share about 10,000 atoms with Shakespeare. Dunno what really bored mathematician worked that out but it's interesting.
Death is like when we were growing in our mother's wombs. One moment we're just a collection of cells and flesh, the next moment the first brain waves happen indicating life. One moment just organic matter, the next life. Same with death but in reverse.
I like to think that in the future, death will be reversible to some extent since it's really just a mechanics type problem. Something isn't working right so fix that and life should be sustinable. Seems most death comes when organs get so old they develop flaws and faults and quit working properly. But if we can replace, or repair them, longevity should be greatly extended. As to what happens to a person when we die, I have no idea. Only honest answer is "I don't know."