We could have included the pybind11 sources as part of our project source code repository, which would simplify the CMake structure and remove the requirement to have network access to the pybind11 sources at compile time. Alternatively, we could have defined the pybind11 source path as a Git submodule (https://git-scm.com/book/en/v2/Git-Tools-Submodules) to simplify the updating of the pybind11 source dependency.
In our example, we have solved this using FetchContent, which provides a very compact approach to referencing a CMake subproject without explicitly tracking its sources. Also, we could have solved this recipe using the so-called superbuild approach (see Chapter 8, The Superbuild Pattern).
