Final answer:
The building in Paris used to detain French Jews prior to deportation was primarily a sports arena, specifically the Vélodrome d'Hiver, used during the Vel' d'Hiv Roundup in 1942.
Step-by-step explanation:
The building in Paris where French Jews were detained before being deported for extermination was primarily used as a sports arena. Known as the Vélodrome d'Hiver, it was infamously utilized during the Vel' d'Hiv Roundup in July 1942, where thousands of Jews were held under harsh conditions before being transported to extermination camps like Auschwitz.
Once at these camps, many were sent directly to gas chambers or selected for forced labor. The use of such detention facilities was a part of the broader context of the Holocaust, which saw the establishment of ghettos, concentration camps, and extermination camps across Europe by the Nazis to fulfill their goal of the 'Final Solution' - the systematic extermination of the Jewish population.