> Is it true that headers in Include/internal are more private than headers in Include/cpython? It is not clear from names.

You cannot include an header file from Include/internal by default. All internal header files are guarded with:

#ifndef Py_BUILD_CORE
#  error "this header requires Py_BUILD_CORE define"
#endif

A C extension must define Py_BUILD_CORE macro or variant to include such header file. There is no intent to tehcnically prevent to include a header file, only an header to warn users that you should not do that unless they understand what they do.