Yes, React Native uses native mobile geolocation.
Although the Geolocation API is exposed through polyfills (Navigator.Geolocation), the GPS signal used is the same as the device.
This already answers the second question (accuracy): the GPS of most cell phones today return their location with a margin of error of only a few meters. Keep in mind that this varies according to where you are using the GPS.
- Inside a concrete building you probably won’t get a reliable location.
- Inside a car, 10-50 meters error margin;
- On foot: 5-10 meters margin.