Two or three components, the military had press with them.
Also, exploiting war crimes by the enemy for propaganda reasons was very important for morale, so it was a high priority thing to have writers and camera people available to document things.
Regarding distribution, specifically the Germans put the info into weekly cinema productions, https://en.wikipedia.org/wiki/Die_Deutsche_Wochenschau But there was also radio and newspapers.
Ultimately it depended on either the military or reporters uncovering it and then reporting it.
Japan did establish Unit 731
Not sure how that was in that case.
edit:
I actually forgot something crucial: There was censorship. The military opened every single letter that came from or was sent to theaters of war. They would let things go through or censor them, depending on whether they deemed something either bad for political or military reasons.
So in many cases there was NO way to get specific news back home.
I am genuinely surprised people still get arrested for that. I suppose protection is only for the super top dogs.
Or that's how they recruit people. Either find those with that kind of history, then make them work by offering, then spring the trap anyway, "for justice".