My father was a POW, and although he didn’t give any great detail about exactly what it was like, it was clear that by and large the Germans treated British, like other prisoners of the western Allies, more or less according to the Geneva Conventions. The camps were visited (not frequently) by representatives of “the Protecting Power” (Switzerland in the case of the UK) and of the Red Cross. To judge by the reports I’ve read in our National Archives on the camps where I know my father was, the Swiss were rather more inclined to believe what they were told by the German commandants, and the Red Cross rather more sceptical and inclined to report complaints by the prisoners (for example, they reported on complaints that anti-aircraft guns had been installed within the perimeter of one camp, and the concern among prisoners in another camp about the way one named British POW was trying to get himself made their “man of confidence” or authorised representative vis-à-vis the Germans - IIRC the name was that of a known prewar Fascist, which the British authorities might well have picked up on receipt of the report).
The Geneva Conventions allowed for “other ranks” to be put to work on civilian (non-military) work, so my father was for a time working in railway marshalling yards in Munich and later in a coal mine in Poland. German and Italian POWs in the UK were frequently put to work on farms, often with minimal supervision, and were held on to for some years after the war (and quite a few were given the option to stay in Britain, and took it up). Officers weren’t required to work.
Food and boredom were a constant problem, but they did get mail and parcels of food, clothing, books, games, musical instruments and records and the like. But these could be intermittent depending on the fortunes of war affecting transport, not to mention the whims of the authorities and individual martinets in the administration. If, for example, it was found (as on occasion it was) that the materials in parcels were being used to smuggle in escape maps and similar equipment, that could trigger off group punishments, as could escapes and other infringements of rules (hence the highest level authorisation for the group escapees from Sagan to be shot as commandos on recapture). And as the war went bad for Germany, so the civil administration went from bad to worse, for POWs as for civilians, and food supplies suffered particularly badly towards the end of the war - the final report of the Red Cross on my father’s last camp in Bavaria in early 1945 is particularly eye-opening about this.
My father always attributed some part of his later problems with his spine and vertebrae to having been hit with a rifle butt by a German guard; and on his return to the UK he reported to the war crimes investigators on an incident happening to someone else, where a guard had got into a scuffle and punch-up with a POW and shot him, the bullet passing straight through and also killing another prisoner behind him.
But there was nothing like the deliberate starvation, indifference to disease and maltreatment, excessive forced labour, and so on, that the Germans applied to Soviet prisoners or the Japanese to just about all their prisoners.