It is quite likely that when the death certificate is made public (if, it ever is), it will simply say "old age" and will be certified by the Royal Physician. The death will have to be registered in the Ballater area (I have no idea of the registration district) and I am not all that sure about the law on the public release of Scottish death certificates. I do not know if there is a separate registration system for the monarch. (I do know that her Will will be sealed and not made public and kept in a safe by (I think) the President of the Family Division).
She looked bright and lively on Tuesday. By Weds evening a zoom meeting was cancelled. Although it was not considered serious enough to call Charles until Thursday morning (when a helicopter collected him from Ayrshire).
I suspect we may never know. But she was 96. And at 96, every day is a bonus!