With the Nest Learning Thermostat 3rd Generation, you can use either the Nest app or the Google Home app. The principles apply to both, but the details vary depending which you use.
With home/away you can use mobile phones and/or certain nest devices to determine whether you are home or away. The best mix depends on individual situations, but I think for most people with just a thermostat using only mobile phones is the best solution.
To start with, only enable your phone(s) for home/away assist or presence sensing and no other devices. Every person's phone in the household should participate. People with more than one phone should generally only enable one. It will use the phone's location services to determine if a particular phone is nearby or not. When all phones are no longer nearby, Away is set. When at least one phone is nearby Home is set.
Make sure location services is on, enabled for the app (Nest and/or Google Home) and works on the phone. It will use cellular, wi-fi and GPS to determine where you are. Those need to be enabled. Make sure your phone accurately places you using Google Maps.
