Building C and C++ Extensions and Building C and C++ Extensions on Windows are the guides to building extension modules.

For most cases, we are able to use setuptools, meson, CMake, or other build systems. However for CPython's own documentation, we should have a cross-platform primer on building extension modules by hand (i.e. without setuptools), likely heavily referencing sysconfig, to guide readers through the internal process of how extension modules are built.