Ok, can you tell me how the learning is accomplished in the absence of geofencing?
Geofencing is no part of it, unless you want it to be, and then AFAIK, you would probably use Android Rules to trigger - I'm not aware of any Nest Geofence capability. What the Nest does have is proximity sensor. It uses it to wakey up the unit when you get near, and to register when people are moving about. It combines that with AI smarts - eg, Mr google will work out when you are going to work - and set to away, if you let it. Also, if you are bought into the Google ecosystem (did I mention I was a fanboi?) you can tell Google assistant "Set heating to away" or "Turn heating off for next 14 days". (not a unique Google feature, part of Teh "Smart" stuffs)
At its simplest level, if you regularly turn heating up or down at a certain time it will learn and try to match, compensating for external temperature.