how would add_subdirectory(pybind11) work?

[jwuttke]

https://pybind11.readthedocs.io/en/stable/compiling.html#building-with-cmake
advises to use the CMake code line

add_subdirectory(pybind11)

• Why not start with find_package(pybind11 REQUIRED)?
• What should be the content of subdirectory pybind11?
• Would the add_subdirectory command work with Debian package pybind11-dev, which has the following content:

# dpkg -L pybind11-dev
/.
/usr
/usr/include
/usr/include/pybind11
/usr/include/pybind11/attr.h
/usr/include/pybind11/buffer_info.h
/usr/include/pybind11/cast.h
/usr/include/pybind11/chrono.h
/usr/include/pybind11/common.h
/usr/include/pybind11/complex.h
/usr/include/pybind11/detail
/usr/include/pybind11/detail/class.h
/usr/include/pybind11/detail/common.h
/usr/include/pybind11/detail/descr.h
/usr/include/pybind11/detail/init.h
/usr/include/pybind11/detail/internals.h
/usr/include/pybind11/detail/type_caster_base.h
/usr/include/pybind11/detail/typeid.h
/usr/include/pybind11/eigen.h
/usr/include/pybind11/embed.h
/usr/include/pybind11/eval.h
/usr/include/pybind11/functional.h
/usr/include/pybind11/gil.h
/usr/include/pybind11/iostream.h
/usr/include/pybind11/numpy.h
/usr/include/pybind11/operators.h
/usr/include/pybind11/options.h
/usr/include/pybind11/pybind11.h
/usr/include/pybind11/pytypes.h
/usr/include/pybind11/stl
/usr/include/pybind11/stl/filesystem.h
/usr/include/pybind11/stl.h
/usr/include/pybind11/stl_bind.h
/usr/lib
/usr/lib/cmake
/usr/lib/cmake/pybind11
/usr/lib/cmake/pybind11/FindPythonLibsNew.cmake
/usr/lib/cmake/pybind11/pybind11Common.cmake
/usr/lib/cmake/pybind11/pybind11Config.cmake
/usr/lib/cmake/pybind11/pybind11ConfigVersion.cmake
/usr/lib/cmake/pybind11/pybind11NewTools.cmake
/usr/lib/cmake/pybind11/pybind11Targets.cmake
/usr/lib/cmake/pybind11/pybind11Tools.cmake
/usr/share
/usr/share/doc
/usr/share/doc/pybind11-dev
/usr/share/doc/pybind11-dev/changelog.Debian.gz
/usr/share/doc/pybind11-dev/changelog.gz
/usr/share/doc/pybind11-dev/copyright

Note that this contains no CMakeLists.txt. So again, how could add_subdirectory(pybind11) possibly work?