Move doc on structure from devguide to InternalDocs (GH-142237)

Co-Authored-By: Paul Ross <apaulross@gmail.com>
Co-Authored-By: Carol Willing <carolcode@willingconsulting.com>
Co-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-Authored-By: Ezio Melotti <ezio.melotti@gmail.com>
Co-Authored-By: Adam Turner <9087854+aa-turner@users.noreply.github.com>

InternalDocs/README.md

@@ -11,6 +11,11 @@ # CPython Internals Documentation
 [issue tracker](https://github.com/python/cpython/issues).
 
 
+General Resources
+---
+
+- [Source Code Structure](structure.md)
+
 Compiling Python Source Code
 ---
 

InternalDocs/structure.md

@@ -0,0 +1,40 @@
+# CPython source code
+
+This section gives an overview of CPython's code structure and provides
+a summary of file locations for modules and built-ins.
+
+
+## Source code layout
+
+For a Python module, the typical layout is:
+
+* `Lib/<module>.py`
+* `Modules/_<module>.c` (if there's also a C accelerator module)
+* `Lib/test/test_<module>.py`
+* `Doc/library/<module>.rst`
+
+For an extension module, the typical layout is:
+
+* `Modules/<module>module.c`
+* `Lib/test/test_<module>.py`
+* `Doc/library/<module>.rst`
+
+For builtin types, the typical layout is:
+
+* `Objects/<builtin>object.c`
+* `Lib/test/test_<builtin>.py`
+* [`Doc/library/stdtypes.rst`](../Doc/library/stdtypes.rst)
+
+For builtin functions, the typical layout is:
+
+* [`Python/bltinmodule.c`](../Python/bltinmodule.c)
+* [`Lib/test/test_builtin.py`](../Lib/test/test_builtin.py)
+* [`Doc/library/functions.rst`](../Doc/library/functions.rst)
+
+Some exceptions to these layouts are:
+
+* built-in type `int` is at [`Objects/longobject.c`](../Objects/longobject.c)
+* built-in type `str` is at [`Objects/unicodeobject.c`](../Objects/unicodeobject.c)
+* built-in module `sys` is at [`Python/sysmodule.c`](../Python/sysmodule.c)
+* built-in module `marshal` is at [`Python/marshal.c`](../Python/marshal.c)
+* Windows-only module `winreg` is at [`PC/winreg.c`](../PC/winreg.c)