[enonod]

I have seen this question in one form or another and the answers leave questions for me.
When metadata is updated manually, say Author, Title etc. I presume an OPF file is lodged with the book in its folder.
I understand (I think) that when the book is saved to disk or sent to a device the opf file data is written into the book file in the same way as exif data in a photograph.
I am presuming that Save to Disk is making a backup to another disk (I don't know if it is simply the books or the whole folder for an author or the whole library.

1. If a book is sent to a device and the opf incorporated into the book file, is it only in the copy that arrives at the device (or disk) or is it the original as well?
2. If a book is requested via catalog is the result the same or only for pushing?

3. If it is both, why is it not incorporated immediately instead of producing opf?
4. If it is only the one despatched, why?

Thank you for any enlightenment.