I don’t know the answer to this but I will say that I believe (and not only me, many other people believe this) that you aren’t your human body. You are God just dreaming that you are human. So when you die, you still exist. This human experience isn’t all that you are. As far as other people existing, I believe other people are just different perspectives of you. You as God could become someone else if you really wanted to, but most of the time you stay in your own perspective (your current human self). So do other people exist? I don’t think so. I think they are all you. But, their human selves are just as real as your human self is, if that makes sense.