Specifically, prisoner exchanges, for lack of better word.

I'm curious about the logistics of it. I've been reading about the Civil War lately and there are often cases of a number of soldiers, be they Union or Confederate being captured. So, for instance the CSA captures say 500 Union troops and then they're later exchanged for a number of CSA troops. How did that work?

I'm aware that both sides had prison camps. Did they "store" the soldiers there and then when needed count out the number they needed and march them to somewhere and exchange them? How much trust was involved? And would it be considered anathema to breach said trust?

And following from that, in Stephen Sears Landscape Turned Red he mentions an episode of a Union officer being paroled and a little later on meeting a column of Union troops and accidentally warning them before remembering that he was paroled and obligated to not take part in the war until such a time as an exchange could be made. But he seemed to be let go, the CSA trusting him to uphold his parole. Did they really do that?

Sorry for asking a bunch of questions instead of just one, but I'm really curious and the do seem related.