Can anyone please point out to me which part of the Subversion source code does actually represent the server portion?
Subversion's server portion relies mostly on Apache, so you are interested in the Apache SVN modules. Those are
mod_authz_svn. The majority (but not all) of the "server" code is in repos.c (and everything it includes). That's a starting point, at least.
If you are interested in learning how Subversion's server works, you'll probably want to get a good understanding of Apache, even take a look at the source there too.