Apple does not officially provide a direct file for macOS High Sierra 10.13.6; instead, they distribute the full installer as an .app file through the Mac App Store.
Before we dive into the download process, let's quickly explain what a DMG file is. A DMG file, short for Disk Image, is a file format used by macOS to distribute software, including operating systems. It's essentially a mountable disk image that contains the installation files for macOS High Sierra.
This is the safest and easiest method. Apple hosts the installer on their servers, but it is hidden behind a "Get" button in the App Store for older versions. You can use a direct link to force the App Store to display the download.
Downloading macOS installation files from third-party websites (e.g., software repositories, torrent sites, or direct download mirrors not owned by Apple) is . Unofficial DMG files may be modified to include malware, ransomware, or backdoors. The methods below utilize Apple's official servers to ensure the file is authentic and safe.
If you let me know whether you need the installer for a specific old Mac (or for a virtual machine), I can guide you step-by-step through the official download process.
If your Mac is newer (2019+), High Sierra will not install. The firmware prevents it.