The Nazi party was ideologically crazy, not arbitrarily homicidal. Hitler, himself was quite the Anglophile and the Nazi attitude towards the US as far as I can gather is that they believed the Jews has too much power but aside from that it was a sterling example of what Aryan people could achieve when lesser races were in their proper place.
AFAIK there was never any perceived need or desire to kill Aryan folk who just happened to be fighting for the wrong side. Even the Stalag evacuation marches described by ftg seem to have been a result of the nasty blend of idealism, incompetence and the chaos of a besieged country rather than any deliberate attempt at killing the prisoners. I don’t know the details but as far as I know those were genuine attempts at evacuation performed by a disorganised and under resourced army in the face of a rapidly advancing enemy. The fact that wounded and ill prisoners were evacuated by train first should be the biggest hint that nobody wanted to kill the prisoners. That doesn’t necessarily justify the move in the first place, but the Nazis certainly had no particular qualms about actively working prisoners to death or simply mass executing prisoners outright as any surviving Russian POW could tell you.
I can’t see any reason why the Nazis would kill allied POWs any more than they would kill French civilians. Provided they belonged to an acceptable race and didn’t cause problems there was no reason why they wouldn’t be treated reaosnably well.
I know it’s very easy to get the impression that the Nazis were homicidal maniacs, (mostly because they often were). However they weren’t raving lunatics, they were raving idealists. The state position wasn’t to kill everyone who wasn’t German, it was to kill everyone who wasn’t appropriate. There’s no more reason to believe the Nazis would shoot all prisoners and dump them in mass graves than to believe the same of the US. The Nazis cared for prisoners because they were human beings. That’s all the reason that was needed. Those prisoners who weren’t viewed as human were treated like animals.
You don’t need to look for any ulterior motive in relatively good Nazi treatment of POWs. They did it for the same reason as the US or England treated their prisoners reasonably well. The fact that they could do that while simultaneously seeing other prisoners as subhuman is IMO far more sinister than simply believing they never kept any strangers alive without a reason.
Anyway, back to the OP. It’s going to be hard to get a ‘general’ overview better than you could get from Google. There are numerous descriptions of POW camps, descriptions of daily life and photos readily available. This seems to be a fairly good example: WWII B-24 Prisoner of War POW Stalag Lufts of the German Luftwaffe
Life wasn’t always like “The Great Escape”, but in air force officers camps it wasn’t too far from it during the early stages of the war. NCOs and enlisted men were not as well treated as the officers, but again treatment usually wasn’t to bad in the early stages of the war. POW camps for other sections of the military generally weren’t as good as for airmen, but once again officers were treated reasonably well on the whole, while enlisted men were treated less well but still generally humanely. Those men unfortunate enough to be put into a mixed POW camp with Slavic and Italian troops were not at all well treated and often got caught up in the brutality inflicted on their fellow prisoners.
As the war progressed the effect of allied bombing, the disorganisation caused by a disintegrating command structure and the transfer of POWs from evacuated camps onto already overcrowded camps further from the front made conditions gradually worse until by the last days all POWs were on starvation diets and wearing clothes that should have been thrown out 12 months earlier. However to put that in perspective, a good many Germans civilians and troops were also starving and ill-clothed by that point.
The annual mortality rate for US and British POWs was about 1%, which is comparable to the rates for German and Italian POWs. That might seem high but you need to consider that many POWs were wounded before capture. That seems to indicate that there was an effort being made to keep POWs alive no matter how unpleasant the conditions may have been. In contrast the death rate for US POWs in the Pacific was around 40%, while some Japanese POW camps interning Australians had death rates exceeding 80%.