Question

I am trying to understand how Subversion stores branch code on the server side. Does it stores a diff of source based on commit id or replicate a source code in the branch?

Answer 1

SVNBook | Using Branches tells us (check the “Cheap Copies” part):

Subversion’s repository has a special design. When you copy a
directory, you don’t need to worry about the repository growing
huge—Subversion doesn’t actually duplicate any data. Instead, it
creates a new directory entry that points to an existing tree. If
you’re an experienced Unix user, you’ll recognize this as the same
concept behind a hard link. As further changes are made to files and
directories beneath the copied directory, Subversion continues to
employ this hard link concept where it can. It duplicates data only
when it is necessary to disambiguate different versions of objects.

You may find SVNBook very helpful.