The official way to download MacOS High Sierra 10.13.6 is through the Mac App Store. However, if you're looking for a raw download, you can try the following:
Once the download finishes, you will find Install macOS High Sierra.app in your /Applications folder. This is the holy grail—a pure, unaltered version of 10.13.6. Macos High Sierra 10.13.6 Raw Download