Welcome to building your first rez package!
This package gets built using 'bez', a very simple python-based build system that comes with rez. It reads the rezbuild.py file, and executes the build function within, passing in build info such as source_path and install_path.
Rez has extensible support for other build systems, and comes with CMake support included. A CMakeLists.txt.example file is provided; to use cmake instead, just ensure that the cmake binary is visible; rename CMakeLists.txt.example to CMakeLists.txt; and delete or rename rezbuild.py. Rez determines which build system to use based on the build file found in the package source root.
When you run rez-build -i, rez uses your package's definition file (package.py) to create the correct build environment, and then runs the appropriate build system's executable within that environment.