http://ciisqbg45nggykdl6rjdrq3wc64csga4vkphu66qsi65mypeitqedoad.onion/faq
They use several methods to try to match IPs with their physical location. Most will simply query the WHOIS server for whichever of the five Regional Internet Registries (ARIN, RIPE, APNIC, LACNIC, AFRINIC) is responsible for managing IP address allocations in that region.