Trying to understand postprocessing ;)

This might be obvious to the trained eye, but where exactly goes the code that is in the documentation about post-processing after the config sample? Starting with const purgecss and ending with a module.export.

Is that something unconnected to Hugo that has to run after the creation process? If so, what exactly is resources.PostProcess doing? I have the slight feeling that the documentation expects us to know something about tailwind that I specifically don’t know (yet)?