I have just updated my old computer to MacOS High Sierra, and when the installation was done, a client of BitTorrent was open, with a torrent for "Install MacOS High Sierra". I have never installed BitTorrent before, implying that BitTorrent was installed w/ High Sierra. Is this true?
Sorry to say this but the only way to get a bittorrent client installed with macOS is if you get it from anywhere but the Mac App store.
Note that macOS High Sierra is free from the Mac App Store. So there is no reason to get it any other way.
On the off chance that you did install from the Mac App Store it is likely that the bit torrent client was installed previously via some other means, possibly as a Trojan or virus of some other type.
If I were you, I would boot into recovery mode, reformat your hard drive and install everything from scratch. Hopefully you have a good uncontaminated backup.