Oh, as to how they do it, the relays are public and lists are available in a variety of places. You can browse them at torstatus.blutmagie.de. So ISPs can easily get all the IP addresses of relays and block them (or watch connections to them). Finding bridge users is more difficult. Either the BridgDB has to be queried from multiple angles, or they DPI for Tor hand shakes. Obfsproxy bridges use transports that look like other kinds of connections, making that more difficult.