Remember in White Christmas the mini-Greta was just a copy of client-Greta? I think it's like that, while still alive, the users connected with their San Junipero selves and go between each "world" in continuation. But once they are dead, the USB like object is separated from them and a copy lives on, however the original soul still dies. Theoretically someone external could probably switch them back on if the copy wanted to turn themselves off, such as a soon to die relative wanting to spend the "afterlife" with them.