[bigpallooka]

I'm trying to decide which method to use when adding multiple books to a library that contains duplicates and also how to deal with duplicates. There are many posts and excellent advice on all sorts of issues to deal with duplicates. Unfortunately, I can't seem to find specific information on what happens during a merge. I understand this is 2 separate issues:
1) What happens during an auto-merge when adding multiple books?

and

2) What happens during a merge initiated by the user when dealing with duplicates?

Specifically:

What happens to fields in the metadata that have differing information contained such as different ISBN or covers or Tags or Comments, etc.?

Is there any way to guide the process outside of what the program is set to do? Would this be a process of the mysterious plugboards?

I am really most interested in knowing what occurs during the process so I can decide if I should manage merging manually and if it will reduce the workload when the source books metadata is of an unknown quality.

When you manually merge books of the same extension (file type) one disappears. Which one? Is it removed (deleted) from the library?

Regards