Python's most popular implementation is cpython which uses the blazingly fast c language.
Python's source code is open-source and can be downloaded from:
https://github.com/python/cpython
You can use developer's guide to explore python:
https://devguide.python.org/ (from left side menu, click Exploring CPython's Internals )

Built-in functions: https://github.com/python/cpython/blob/master/Python/bltinmodule.c (scroll to 1950 for print function's source code)
Built-in Types: https://github.com/python/cpython/tree/master/Objects
builtin type int is at Objects/longobject.c
builtin type str is at Objects/unicodeobject.c