ament_cmake vs ament_python?

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Is one meant for c++ and the other for python?

Yes this is the main reason.

ament_cmake can be used for CMake package (usually C and C++), and provide some CMake magic to make things less verbose and more convenient to use with the ROS tools. It tells the build tool to build the package using a cmake workflow: cmake -> make -> make install. Some examples of packages available here

ament_python it is used for pure python packages and tells the build tools to use a setuptools workflow: python setup.py -> python setup.py install. Some examples of packages available here

I didn't find any explanation about the difference between both

It looks like a good area of improvement for the tutorial, as it should likely introduce these types in the introduction.