The blank area is there because the template shows the full canvas and the safe area where the important artwork should stay. It isn't a small export size: use the whole canvas for the background, but keep logos, text, and the main subject inside the safe area so they don't get cropped on different devices and orientations. The preview in App Store Connect is useful for checking this.
The export depends on the placement. Apple lists product page header images as 3840 × 1646 (21:9) or 5244 × 2950 (16:9 PNG). Search result images are 1920 × 1280 to 3840 × 2560 (3:2), or 5244 × 2950 (16:9 PNG). JPEG or PNG is fine for the listed image sizes, and the file must not have transparency. If you're using a video, the formats and lengths are different, so check the specs for that placement.
I'd use the export settings in the matching Figma, Photoshop, or Pixelmator template rather than cropping the canvas to the safe-area box. Apple's asset best practices and creative asset specifications show the safe areas and exact sizes.