Manages display.
Acquire the root window then talks to X11 to manage windows creation/remapping/destruction
Also has the ability to manage child processes via xdg-apps
A child process is considered "dead" when it's original PID is killed and the created window doesn't exist anymore.
fontconfig is used to provide fonts (but the program should not crash in its absence).
python and build-essential are required at install time for node-gyp to compile abstract-socket.
A good example of window manager code is Openbox in C.
Otherwise, the Xlib documentation might be of use.
The ICCM spec provides a lot of implementation details that are loosely followed to various degrees.