You're unlikely to ever notice it - satellites generally have shielded electronics in addition to the Earth's magnetic fields to help protect them. Additionally, the Van Allen belts offer more protection the closer to the earth in orbit (lower altitudes), and the most if your satellite happens to be in orbit on the backside of the earth when a flare hits. GPS is medium earth orbit, and has decent protection due to this altitude (12,540 miles above sea level). Solar flares can cause momentary havoc, but they have to be massive and push the Van Allen belts enough to expose the satellite. Even then, shielded electronics and automatic hardware/software reboots mitigate negative effects, and satellite operators can temporarily shut down a satellite at a pre-determined position/duration for further isolation.
GPS works by sending a radio signal of an on-board clock on the satellite, tracking the time between signal send/recieve, to create a "sphere" of consistent distance from the satellite - the device is somewhere on the surface of this sphere. Add a second satellite, and now you have 2 interconnected spheres, and the device is located somewhere on the intersect points. This continues as more satellites are able to be received, with increasing the fidelity of location with each additional satellite. This works by having a satellite in a known positional orbit and a known transmission time - if either of those are missing, then the location can't be determined. The signal sent contains the orbital location and time sent, and travels at a known constant (speed of light), which determines how far those "spheres" are from the satellite's original location at time of transmission.
Modern cellphones are capable of functioning with different positioning networks and cross-referencing them to obtain better coverage and accuracy. My cellphone is capable of using GPS, GLONASS, Galileo, QZSS, and BeiDou systems (American, Russian, European, Chinese, and Japanese).
Cell tower triangulation can only function if the tower's locations are actually known, and the device would need a directory of tower locations onboard to calculate a location of the mobile device, and this is why triangulation location via a mobile device to figure out where it's at doesn't happen. The reverse is possible using tower provider networks, but has limited applications in non-emergency situations.
Geofencing works off of wifi for most of the applications you'll use. The pairing software will function one of two ways, off of Wifi signal (if you're out of range, you're not home, and the lights turn off, for example), OR it pairs a home address into the navigation system on your cellphone. The navigation system is generally functional when you allow the phone to track it's location while applications are not in use, or you allow tracking while an application is in use and you are using it.
In the first case, satnav is not used at all, and this is common for home automation depending on the system. The second case is common with things such as Google home, but only when you're also using Google maps to navigate and it's goes "you're almost home" and the lights turn on. The wifi is still built into it at this point, and defers to the first example as a default. It is possible for meshing to happen depending on privacy settings on the device and multiple wifi's with inputed street addresses to function for geomeshing as well.
The eye roll was based off commentary where the phone was set to not need unlocking at home - and then the poster commented how cell connection is turned off and the home wifi is used instead. Which literally means that the geofencing for that activity is based on their home wifi. Correlation is not causation.
But I'm sure I know nothing about satellites and navigation requirements, I was just a foreign space program analyst for 7 years.
