Just an update on the above. AP will take a route (multiple waypoints) from a GPS and follow the route - however I believe that no AP will make the next turn for you. It will notify you when it is time to accept the next waypoint but, in case there are reasons the turn should not be made, it will not do it for you. That is certainly the case with my dual set up.
To clarify, an AP won't use multiple waypoints. It steers to one waypoint, and that is only a virtual waypoint. In reality, the chartplotter sends a bearing to the AP for it to steer, sends cross track error,(how far off in distance the vessel's current position is from the rhumb line that the chartplotter has determined from current position to the active waypoint), direction to steer (L/R), lat/lon, active waypoint ID and some other data depending on the chartplotter. The AP may receive the lat/lon, speed, or other data but it may not necessarily do anything with that data. The AP has an electronic compass or input from one, that it uses to determine heading, it typically sends that data to the chartplotter. Each AP uses slightly different variations but most are of similar function.
Simrad will make waypoint course changes contingent on a configured setting, typically a course change of less than 30 deg will be made without user intervention. Raymarine OTOH requires acknowledgement for each waypoint course change.
The reason a chartplotter using a route or even a single active waypoint is more accurate is that it continually adjusts the data sent to the AP to compensate for set and drift, something the AP is incapable of. Sure, the AP will steer a compass course, but it's a dumb setting, e.g. it maintains the heading, it just doesn't know where it leads to, so it can stay on the correct heading but end up way off the mark.
It's simply astounding how many cruisers are intimidated by the chartplotter/AP interface to the extent they won't utilize it; even more so how, when they finally get over the hump in the learning curve, they wonder why they didn't do it sooner!