Since at least macOS Mojave 10.14.1, the macOS installers you download from the Mac App Store are lightened versions which doesn't contain all files needed for a complete system install. Those files are downloaded when you actually start the installation.
This tool use an Apple catalog file to download the needed pieces, put them together, and give you a complete working macOS installer.
There is no patching, no hacking, no trick, it just reconstruct an installer from its scattered parts.
