ISS Detector basically works like any other satellite tracking program. It is based on the SGP4 calculation model. There are several SGP4 programming libraries available in different languages. SGP4 is a set of formulas that use a TLE (Two line element set) to calculate the position of a satellite at any moment. The TLE is a set of parameters that describe the orbit. The TLE is published on several websites for example space-track.org. The extra steps involved are calculations to see if the satellite is above the horizon and visible. And if the satellite reflects enough light to see it.
Main components which play a vital role to detect the precise location are compass, gyroscope, gps.
Apps like sky guide, don't use any sort of Google API though. There's a site called Celestrak that publishes the orbits of the ISS and various other satellites. Here's the main site:
The first three lines of this page describe the orbit of the International Space Station:
http://celestrak.com/NORAD/elements/stations.txt
