View desktop viewport within a window in macOS

Simply opening your Desktop folder in Finder is about the best you're going to get.

It won't be organized like your actual desktop unless you organize it manually, but think about this: if you actually could have a desktop viewer, how would it be laid out? Not like your desktop, because it would only be half as wide. So it would have to reshuffle the files and folders anyway. Alternatively, you could change the spacing and icon size in your Desktop folder to 1/2 of their current settings, allowing you to display an exact replica of your desktop in one-quarter size (half-size in each dimension).

You can even set the background of the Desktop folder to your wallpaper by finding it in /System/Library/Desktop Pictures/. Example

Edit: To use your wallpaper as the background image in the Desktop folder, you’ll need to drag either the light mode or dark mode image somewhere (this will create a .tiff copy) and use preview to resize it. 30% of original size works well for a quarter-size Desktop window.