Well for torrent it starts by contacting one of the various trackers that know other peers. It's not centralized but there's only a couple of trackers out there

Well for torrent it starts by contacting one of the various trackers that know other peers. It's not centralized but there's only a couple of trackers out there. There's no trust between any of the peers, but each of the torrent piece has associated hash, meaning peers cannot give you invalid data without being caught (unless hash collision occurs). Peers can be discovered with DHT magic, but ultimately, can only be dialed if the ISP allows peers to receive connections.