When you install a *.src.rpm, it is installed under /usr/src/redhat/ (/usr/src/packages/ on SuSE). A source tarball(s) and patches are installed in the SOURCES subdirectory, and a <package-name>.spec file is copied to the SPECS directory.
You use the rpmbuild command to build or install from source. There are different stages, and the rpmbuild options let you stop at any stage. For example, suppose you already have the package installed but you want access to the patched document source.
You could run "rpmbuild -bp <package-name>.spec" to apply the patches.
To apply the patches and build new binary and source packages, run
rpmbuild -ba <package-name>.spec
To apply the patches, compile the source, and install the package run:
rpmbuild -bi <package-name>.spec