I have downloaded a genuine copy of macOS High Sierra from a website and after I opened it I can see many part files (like part 1, part 2 ...). How can I install macOS from these part files? How can I make a bootable out of it?
It looks like the installer came in archive parts. You can use The Unarchiver from the Mac App Store to extract the installer from the archive. Be sure to select part1 when extracting. Also remember, downloading macOS from 3rd party websites might be dangerous as they might be modified.
To answer the question in your title. You can create a bootable MacOS High Sierra disk without downloading it from the Apple store.
Using your Apple ID, create a free developer account and download the installer from https://developer.apple.com/download/
I certainly would not be installing a copy of macOS that wasn't downloaded from Apple