German manpower on the Western front in '44 was a mixed bag. Regular Wehrmacht, SS, a lot of Hitler Youth, volunteers from occupied countries, Russians who were willing to fight for Germany to get out of the camps... there was an odd mix. There weren't as many first-line troops as you would think. They were needed in Italy and on the Eastern Front, and reserve and second-line units were suitable for occupation duty and for manning fortifications, so they were used to free up that manpower.

All in all, Nazi manpower there at the time of D-day consisted of everything from French collaborators to teenage German paramilitaries to Russian "volunteers" to hardened combat troops

Most of the Belgians who joined the SS ended up at the Eastern front.

In general they quickly discovered that the German propaganda did not really resemble the reality. They were seen as second-class soldiers by the Germans and used as cannon fodder. Nevertheless they mostly remained loyal to Germany during the war.

I only have numbers for the Flemish but of the 12.000 that joined 5000 died and many many more were wounded. I know of a particular group that left with 1.200 soldiers and came back with 37.

Those that did fight on the Western front or were aiding the German troops/government were prime targets for the resistance groups.

Taking D-Day/Overlord as a specific example, the defenders were mostly German (including many second-line troops, such as 'ear and stomach' battalions), but not entirely. Roughly one-sixth of the German Seventh Army (responsible for the coastal defence of Brittany and Normandy) consisted of Ost battalions, largely recruited from prisoners and volunteers from the Soviet Union including Georgia, Armenia and Azerbaijan, amongst many others; there's quite a famous case of at least one Korean captured by US paratroops who had been impressed by the Japanese to fight against the Soviet Union, subsequently captured and sent to fight the Germans, then captured again, ending up in Normandy.

There were western/northern European volunteers (e.g. Dutch, Belgian, French, Danish and Norwegian) fighting for Germany, but these were predominately in the Waffen-SS operating on the eastern front (volunteers tended to be ideologically motivated fascists/anti-Bolsheviks). Citizens from occupied countries were used as forced labour in Germany and across Europe, but I'm not aware of them being conscripted into the army en masse.

Meanwhile, on the Allied side, there were also combatants from the occupied nations; the RAF, for example, included French, Belgian, Norwegian and Dutch squadrons, as well as individuals in regular squadrons. There were also the irregular activities of resistance groups.