I think maybe i get your question. I believe you're asking:
"How does navigation software decide which task you should do at any point? Is the specific maneuver task hardcoded into the map, or does the device itself decide which task it should tell you to do?"
If that's the question then the answer is almost entirely the device (or server the device is connected to) decides on the fly. There are generally no navigation instructions hardcoded into the map data (unless there's a particularly tricky intersection). The navigation software will simply be programmed to use specific task words for each angle of an intersection. Like:
If route angle = -5 to 5 degrees, then say "turn right"
If route angle = 6 to 85 degrees, then say "bear right"