The letters from your ISP have nothing to do with them monitoring your traffic. When you torrent, you're connecting to a public network of seeders and leechers. Copyright holders pay people to monitor that public list of IP addresses, and they record your IP (because you connected publicly, in the open, and uploaded or downloaded). Then, they send your ISP a letter reporting that your did an illegal thing, and asking them to punish you. Finally, your ISP sends you a letter making some vague threats and asking you to stop. They might make you do a training course to educate you on why piracy is bad, and they might cut off your internet until you pass a quiz and promise not to pirate stuff again. They go through this charade not because it actually accomplishes anything, but because they don't give a shit, and they're just doing the bare minimum to keep lawyers off their back.