I was this many years old when I learned that tar and zip on linux have an rsync compatibility mode that tries to do some cleverness with compression blocks to make it easier to diff two archives.
I thought they were xml?
XML files in a zip file (plus images and other things). There are many individual xml files in a single zip file, each file (part) is generally responsible for different area, e.g. one file for one sheet cells, one file for style definition, one for comments, one for workbook structure and so on.
The whole structure is called Open Packaging Conventions and it is implemented in `System.IO.Packaging`.