An .skp file is a SketchUp document. A SKB file is a backup document created by the Windows version of SketchUp only. SKB files appear as [filename].skb. The macOS version autosaves models with the ".skp" extension as [filename]~.skp. The file structure is not different from that of a regular.skp file, but to open it, it may be necessary to change the extension to .skp.