[lucideer]

> I don't know the ways this data is acquired

It's been acquired from a range of sources. Most notably, Google Streetview cars collected a very large initial data set. Once you have that, it's quite easy to maintain an up-to-date dataset by verifying new routers picked up against nearby known routers (as well as verifying against GPS sensor data obviously).

This tech is not only in Android phones, but also every Chrome browser, so that's a lot of incoming data. The API used to be used by iOS and Safari (before Apple Maps) and Mozilla Firefox (before they launched their own WiFi and cell network database; theirs is released under CC0 though so I'm sure Google use their data too).

There are also other competitors to Google, Apple, Mozilla here with their own databases, like Skyhook, Navizon, AlterGeo.