How does precise GeoLocation in HTML5 work?

whether they also store a huge database of wireless AP points,

Well, that's one way of how it works, according to their blog:

Google Maps asks your web browser for your location. Typically, your browser uses information about the Wi-Fi access points around you to estimate your location. If no Wi-Fi access points are in range, or your computer doesn't have Wi-Fi, it may resort to using your computer's IP address to get an approximate location. As you'd expect, the accuracy of My Location varies with your location, and in some cases, Google Maps may not be able to provide a location at all.

W3C Geolocation API, however mentions

The Geolocation API defines a high-level interface to location information associated only with the device hosting the implementation, such as latitude and longitude. The API itself is agnostic of the underlying location information sources. Common sources of location information include Global Positioning System (GPS) and location inferred from network signals such as IP address, RFID, WiFi and Bluetooth MAC addresses, and GSM/CDMA cell IDs, as well as user input. No guarantee is given that the API returns the device's actual location.