Is the Detection of the Photon Really Telling You About the Interaction?

I'm just trying to relate the art of photonics that again, I know nothing of to detecting these photons. So I guess my question is what is that detection telling you? Or did I just get that completely wrong? Is that, is that not what you're doing? What is that photon really doing in the first place? The technique which I said is for saying whether we have one photon or more than one photon. But this photon can be used eventually for say, for example, for quantum, like single photon level interactions.