The files and links in this folder are populated by the WSJT-X CMake build script. There are install commands which are only run on Apple hosts, this is important because they will get installed at the install root on other platforms, which would be very bad on Linux for example since that is /usr normally!
The symlink to /Applications, the background image (derived from ".../artwork/DragNDrop Background.svg") and, the custom .DS_Store file (".../Darwin/wsjtx_DMG.DS_Store") are all handled specifically by the CPack DragNDrop packager so you don't need to install those.
The DragNDrop installer is a generated a DMG file that has a custom .DS_Store file that defines the layout, background image and, folder view options of the DMG root folder.
To modify this file, first you need to make a DragNDrop package then mount the DMG file, then modify the root .DS_Store file using Finder. Once you are happy with the results, you check into source control the modified .DS_Store file and then future package builds will use that file.
The installer DMG is read only and shrunk to exactly the size of the contents, also the .DS_Store file is read only and the background image PNG file is hidden. You need to undo all of these things before changing the .DS_Store file. Don't forget to redo these things before checking in a new version of the custom .DS_Store file.