Curious to know what and h ow does third party check applicants!

[Torki]

I would appreciate someone clear this for me. Just curious. I am not talking about why though it has another discussion.
Someone got visa recently and came to NZ and after a year wants to apply for residency. I understand countries like China, Iran, Cuba and like these are restricted with a little info to achieve for security officers. However, I want to know exactly what does it mean by third party?!
My officer told me it might take substantial amount of time. Ok I see but i believe this is an excuse for just giving the residency late. If someone got visa recently and one year all has lived in NZ without leaving then why need third party as already did for visa?!
Anyway, how they check specifically in those countries. I am sure I am clean at all and this is why I asked this question as being curious. They are spies? They are working inside those countries government? If someone is the citizen of those countries understand what I am talking about and how it is hard to get info or even mis info!
If someone hires by a government it is so easy to do it without any sign.
Anyway, who are they and how they get info?

Again sorry for asking here as I am curious and a bit sad! Though I respect all the rules but this makes life hard actually!