In the immediate aftermath of the Fall of France, around 2 Million French soldiers were captured by German forces. Germany kept a large majority of French soldiers in POW camps in either France or Germany. The majority of them were kept in POW camps in Germany for the explicit purpose of using them as forced labor, as Germany experienced severe labor shortages in manufacturing, mining, and agriculture due to their mass mobilization. Non-white French soldiers (Algerians/Moroccans, etc) were usually kept in France rather than Germany to help construct military projects like the Atlantic Wall.

Some were given the option to join volunteer units, such as the Charlemagne Division of the SS, once fighting in the Eastern Front began rather than becoming laborers. Ironically enough, this same SS unit made of up former French POW's and right-wing volunteers was the last unit to defend the area surrounding the Führerbunker when it finally fell to Soviet forces on May 2, 1945.

The remainder were either kept in camps without being put to work(mostly officers and NCO's), escaped, or were repatriated to France after a brief period of captivity for various reasons.

Source: Hitler's Empire by Mark Mazower