No it's not the tracker, it also works for magnet links and people get letters for downloading.
There are companies who join the download swarm and register all other downloading parties. That's very easy with bittorrent, since the protocol is (originally) designed for fast download sharing without any regard to anonymity or pseudonymity.[1] The process is not reliable for providing evidence of copyright infringement, though, and the German system mostly works by scare tactics of lawyers - many people don't want to risk a lawsuit even if they could win it.
There are companies who join the download swarm and register all other downloading parties. That's very easy with bittorrent, since the protocol is (originally) designed for fast download sharing without any regard to anonymity or pseudonymity.[1] The process is not reliable for providing evidence of copyright infringement, though, and the German system mostly works by scare tactics of lawyers - many people don't want to risk a lawsuit even if they could win it.
[1] https://torrentfreak.com/thousands-of-spies-are-watching-tra...