If you want to modify the source code of the version that you have installed, then using apt to install the source code is the best way. You can use git or download the source code of many programs, and even get newer or alternative versions of software like this. However, the source the Debian uses is likely patched by Debian to some extent. That means Debian has added some minor adjustmments to the code, to optimize it on Debian. You can read about using apt to install source here:
https://wiki.debian.org/apt-srcThe next step is you need to have the toolchain and compiler for source code. Rhytmbonx and all GNOME software are written in C and they use many GNOME libraries. SO you need a c compiler, which almost every linux distribution comes with. There are also packages that are in Python, or other languages. After you make the modification to the code, you need to compile it and preferably make a .deb package. After this, install it. There is a post on this forum called "how to compile a package the smart way". You should read that.
With GNOME packages there is also a very easy way to get started. You can install a programming IDE called GNOME builder. This is an IDE that can be used to write C, C++, Python or many other languages. It also, however comes with ready-made templates for GNOME programs, if you want to make your own (things like documentation and file trees and libraries are utilized in a standard way). In addition to starting a new project it allows you to open the projects of other GNOME software. So you can just start editing Rhythmbox by selecting that from the list when you make a new project.