What are the shadows in the four corners of the photo?

The reason is that the front edge of the lens or the hood blocks the lens, and sometimes the reason is that it will be blocked at the wide-angle end, and it will not be blocked when it leaves the wide-angle end. If your lens has a hood, when you shoot at the wide-angle end, remove the hood so that the corners will not be dark.

If your lens doesn't have a hood, try to shoot only with a small aperture, which will also reduce the darkening of the corners.