X-Plane 11.30b1 includes updates to the default Cessna 172 autopilot. It is now an S-Tec 55, which is a high-end general aviation dual-axis rate-based digital autopilot. This is a big change from how the C172 autopilot worked previously. Most notably, it does not have buttons with toggle logic, so you cannot press the button of an active mode to go back to a default mode. You always have to select a new mode to cancel an old mode. In addition, this autopilot supports GPSS by pressing the NAV button twice.
In this video tutorial, Philipp Ringler provides a thorough explanation of how to use the new autopilot and its various features.