Essentially any android or iPhone with location services active will periodically report SSIDs and the current location.
This is put into a publicly accessible database/API to allow location lookup based on nearby SSIDs that’s accurate to a few meters.

“You may not have Apple products, but if you have an access point and someone near you owns an Apple device, your BSSID will be in [Apple’s] database,” he said. “What’s important to note here is that every access point is being tracked, without opting in, whether they run an Apple device or not. Only after we disclosed this to Apple have they added the ability for people to opt out.”

Appending _nomap to your wifi ssid apparently opts-out of Google and Apples WiFi positional service database