How do photographers for Newsweek/other news magazines get those pictures of soldiers in Iran/Iraq, etc?

They are usually embedded within a squad just to take pictures. Soldiers usually don’t mind.

They are actually there with them, as @Fiddle_Playing_Creole_Bastard said. It’s a very dangerous job.

Here is an article that you might find informative, troubleinharlem.

And here are some additional sources. While they’re aboout the Vietnam War and deal about the media in general and not only photojournalism, it gives a good idea of its significance.

They are war correspondents. Just as Hemingway was.
And they are as at risk as the troops they travel with.
List of journalist deaths in wars since WWI
Death of embedded Sunday Mirror [UK] journalist in action, Afghanistan