"If I am not mistaken the GPS feature works as part of the 4G/cellular connectivity, so I guess it won't work on the open where there is no cellular reception, am I right?"
Five year ago, that could be said of most or all of the phones on the US market. While the rest of the world was using phones that directly accessed and resolved GPS data, the US cellular carriers (the carriers, not the phone makers) all required the GPS sets to be crippled in the US version of the phones, so that US phone suckers, ergh, customers, had to pay an extra $10 per month to the carrier to use the GPS data.
That's not true at all.
The calculations to get an initial GPS position fix are very complex. If you had a dashbord GPS navigation unit from the mid-2000s, it took a minute or two to do those calculations. Even on the 2010 Garmin I bought my parents, it takes about 30 seconds. Phones, which are designed to stay on for 16-24 hours vs. a few hours for a handheld GPS, simply didn't have the processing power to do those calculations in even a couple minutes.
Rather than force you to wait a a few minutes every time you turned on the GPS, the phone companies came up with assisted GPS. In A-GPS, the phone transmits the GPS satellite data to the tower, a big computer there does the difficult calculations in a couple seconds, and transmits the coordinates back to the phone.
Then the carriers found out they didn't have enough data capacity to deal with a stream of GPS requests, so they allowed US-market phones to carry independent GPS capabilities.
The GPS data needs to be sent to the tower only for the initial position fix (when you first turn on GPS in a new location). After the phone has the intial position, the relative position changes are a lot easier to calculate and are done on the phone. Even an early-2000 phone can do those calculations on its own in real-time. Once the phone had its initial position, you could turn off its data and (provided you had the maps pre-loaded) it could track your movement just fine. That's how I used my 2004 flip phone with A-GPS to navigate to a destination in Canada, where I had no data service there. I just made sure not to turn the navigation app off after I'd crossed the border.
So A-GPS has almost no impact on data capacity.
I'd assume that means the older iPhones were all carrier-compliant and required AGPS, while the newer ones certainly do not. Maybe the change came in around the 3rd generation?
Processors are a lot faster now and the phones can calculate the initial position in a reasonable amount of time.