Unity AI – Reinforcement Learning with Q-Learning – Unity Blog

A* finds the optimal path if the graph is known (you can describe exactly each state). Q-Learning (and Reinforcement Learning in general) tries to find the optimal path under unknown circumstances (part of the algorithm is to discover possible states, and often there are so many combinations that you can't learn all of them anyway) and in stochastic environments (action only leads to expected state with a certain probability). If you know the map of your country and want to navigate between cities, use A*. If you want to find the optimal order of steps (keystrokes) in a game, it's impossible to describe the problem as something A* could solve. Remember that A* uses heuristics to determine the next action.