The guesses can get scarily good. Typically you start with the GNSS signal as your initial position/velocity estimate and inertial measurements to estimate travel from that point. With decent sensors and no other constraints, you can get lane level accuracy for upwards of a minute at highway speeds without a satellite fix. However, people (and vehicles) don't have arbitrary paths and the places where they do tend to be easy to get a satellite fix. The kinds of places where you have difficult GNSS environments also tend to have networks of paths and streets that can be used to refine your estimate., a technique called "map matching". In good conditions, you can localize position to within a block inside an entire urban area on that alone.