Not withstanding Sygic's over-the-top claim to be first, there are two types of GPS navigation systems out there. I used to write professional articles on GPS systems when they were first coming out, when the accuracy of GPS was permitted to be used by civilians by the DoD to a level where it was practicable to use as a navigation system for cars.
If you want to know more about how it works, keep reading, otherwise you can stop now.
The two types of GPS navigation systems are what I will call on-demand and on-device. The difference between the two are where the maps are stored and where the navigation is performed. On-demand systems are dynamic in that they store the maps at some location in the cloud, downloading what you need for the route you ask for when it is planned. The planning can be done either at the mothership (as in Waze) or on the device. So the on-demand approach requires connectivity to the mothership, either by cell or wifi. And what happens without the connection varies. Waze, for example, plans your route and downloads the maps you need for that route at the start, so that it can show the entire route for you. And as you navigate, it constantly updates for traffic, obstructions, police, etc, etc. If you lose connectivity, it will use the last calculated route for you but it will tell you it has lost that connectivity and it won't update the route for those outside factors. Nor will it recalculate if you leave the planned route, as all planning is done at the mothership. I have a route I use it for fairly frequently and there is a 20 minute stretch with no cell service, so Waze complains for that 20 minutes. Once back in coverage, sometimes it automatically connects, sometimes it seems to have given up and won't connect unless I stop it and restart it. I suspect Apple and Google maps work the same way.
The on-device navigation systems are, as it sounds, on the device and can be used with zero external connectivity. Of course, if you want traffic, or dynamic routing around accidents, incidents, construction zones, etc., you will need some sort of connectivity. So when Sygic, for example, offers traffic updates, it is assuming you have that connectivity. No connectivity, no avoidance navigation.
So which is better? Depends. On-demand requires connectivity for just about everything. Although the "local" map may be on your device because that was the last one you used, if you use On-demand and want to go someplace new, particularly if it's far away, those maps for that route will need to be downloaded for you. But the advantage is that you get the absolutely latest information, including traffic delays, in near real time. On-device has the advantage of working without any connectivity at all, but you have to dedicate storage for the maps you *might* use, even if you aren't using them very often. Right now, I have an on-device navigation system called CoPilot GPS with all the maps for North America (US, Mexico, Canada) and it takes 3.61GB of storage. I also have Waze and it is reporting 108 Mbytes, or about 3% of what CoPilot is using.
As for accuracy, GPS for cars is accurate to about 5 meters (16.5 feet). That's good enough to use on the road as most of the system use a feature called "snap to road" that assumes you are ON the road, even if the most recent fix is not. Theoretically it is possible to have accuracy to within millimeters, if you want to step up in price to a dedicated GPS system. (Think Google Maps and Apple Maps cars driving around. I suspect they have a more accurate GPS to get the maps as accurate as they can.) (Geologists also track movements of the earth along earthquake faults using high-accuracy GPS receivers.) Cell tower triangulation can help get a bit more accuracy, which is why you see that nag notice if you turn off cellular and just use GPS. The improvement with cell triangulation is to about 5 feet, which is handy in cities where the bouncing signals from GPS can make it less accurate. I suspect 5G will enhance that triangulation, but I haven't seen any figures on it.
GPS is degraded if the satellite "constellation" above you is out of optimum shape. When a satellite gets low in the sky the bending of the signal through the atmosphere can make a few nanoseconds worth of error. (A nanosecond of error is just under 1 foot, so a delay of 15 nanoseconds would give you an additional 15 feet of error, for a total of 30 feet of error.) Bounce is even worse as every foot of added distance for the signal in the bounce degrades your position by that same foot. That's why GPS wanders around in cities as the signals bounce all over the place. Recent launches of additional satellites has significantly reduced the "bad constellation" instances and the logic in the calculations says, basically, "5 of us agree we are HERE and you single satellite vote THERE. We will use HERE."
Some systems will let you view the GPS data as it comes in. "Back in the day" you could actually see the constellation of satellites being used for the fix, so you could judge for yourself the accuracy. I haven't done that in years--no need for it with all the satellites that are currently up.
If you want more boring tech details, PM me. No need to bore the rest of the visitors to the forum!
EDIT: Added a bit of clarity on Waze navigation being done on the mothership.