Just to be 100% certain, I downloaded the 2018 installer from that link. It is a dmg file. You open that dmg and it will display a dialog with two icons, one for SketchUp and one for the Applications folder. You install SketchUp by dragging the SketchUp icon onto the Applications folder icon in the dialog. That will copy the files to the correct places. Then close the dialog and eject the dmg. All other setup is done when you then open the apps from your Applications folder.
I suspect you are navigating to the dmg in Finder and looking around inside it. That is not the right way to install or access SketchUp. In fact, if you try to run it from inside the dmg, all sorts of errors are likely because that is not the right location!