He probably saw a photo of Germania. The arch is in the middle of the photo.
[Photo caption] Model of Adolf Hitler’s plan for Berlin formulated under the direction of Albert Speer, looking north toward the Volkshalle at the top of the frame
Welthauptstadt Germania (lit. ‘World Capital Germania’) was the projected renewal of the German capital Berlin during the Nazi period, as part of Adolf Hitler’s vision for the future of Nazi Germany after the planned victory in World War II.
He probably saw a photo of Germania. The arch is in the middle of the photo.