Dependencies for Building GNU Radio from Source


I’m trying to understand the GNU Radio software architecture and build
system. I have a question about dependencies. It says on the GNU Radio
Wiki pages to ensure that the dependencies are satisfied before
attempting to build form the source tarball. My questions are:

Where are these dependencies called or referenced to, from? At which
stage of the build process are these dependencies required? Does the
C++ code need them ,or is itthe SWIG part of the build system?Are
libraries from these dependencies built into the shared libraries that
SWIG generates? Is it so that these dependencies are only required
while actually using the built packages i.e while working with GNU