Discovering hardware devices from mobile app


How do professional hardware devices able to connect to mobile app without first being on the same local wifi network?

Like Sonos, even though it is a Upnp device, its app first asks to turn on location services and then go on to configure the Sonos with the local wifi network.

The app also would not discover other upnp devices as well.

How do they do that?

I dont know how devices like sonos, Google home or Philips hue work but only their respective apps can discover them.

What is going on behind the scene? What protocols do they use for the first time discovery phase?

Why do they ask for location services and how does that transmit unique information about their product?

For example, if I have to connect a raspberry pi to my wifi for first time, i use a temporary hotspot which transmits only ssid.

If I select that network in mobile , it prompts me to enter my home wifi in a captive portal.

I can have a similar pi with similar ssid and that can be confusing.

From what i have come across, upnp (or other cast/openhome) devices need to be on the same network for discovery.



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