Here is helpful text regarding downloading and using macOS High Sierra 10.13.6, including safety tips and legitimate sources.
Backup Your Data: Before starting, it's crucial to backup your Mac. Use Time Machine or any preferred method to ensure your data is safe. download+macos+high+sierra+10136+exclusive
What doesn't work: Any Mac introduced after 2017 (iMac Pro, 2018 MacBook Air, etc.) cannot boot this natively. For newer hardware, you're looking at a VM. Here is helpful text regarding downloading and using
Performance and Compatibility