Jon, you are getting things all mixed up.
A number of years ago the FCC required that all cell phones be capable of providing accurate position data to assist 911 operations. I think they only required something like 100 foot accuracy, and that can be acheived through things like cell and wifi signals in most cases. So initially, some companies used these methods since it could be done with software alone.
But they quickly started to put GPS chips into the phones afterwards. It created a huge new market for GPS chip makers since there are literally hundreds of millions of smartphones sold each year. The early phone GPS chips were rather wimpy, but the technology has greatly improved since then and they are pretty comparable to a standalone GPS now.
Now the issue with Google Maps is not related to knowing your position. Google Maps requires a data connection to send the map itself to you in realtime. It is sent as small "tiles" that only cover the area you are currently in. I think you can download areas in advance on the Android version, not sure about iOS. Not much of a Google Maps fan anymore myself….
A dedicated Garmin device has some advantages, of course. It's shockproof and waterproof, can run on regular AA batteries and they have screens that don't need a backlight during the day. The water and shock proofing can be done with a good phone case (maybe $50) and battery runtime can be greatly improved with a case like
@SuperChooch uses (around $100). The transreflective screen is arguably the main advantage of the dedicated unit, they look at their best in direct bright sunlight. This can be a challenge on a phone, but the screens have gotten a lot brighter recently.
And the other thing about a Garmin unit is that it's more of an "appliance" that's easy to use and manage. Just turn it on, go for a hike, turn it off and put it away until next time. Apps and phones require a bit more "care and feeding".