Q-Learning for continuous state space
Reinforcement learning algorithms (e.g Q-Learning) can be applied to both discrete and continuous spaces. If you understand how it works in discrete mode, then you can easily move to continuous mode. That's why in the literature all the introductory material focusfocuses on discrete mode, as it's easier to model (table, grid .., etc.)
Supposing you have a discrete number of actions, the only difference in a continuous space is that you will be modellingmodeling the state each X$X$ amount of time (X$X$ being a number you can choose depending on your use case). So, basically, you end up with a discrete space, but probably with an infinite number of states. You apply then the same approach you learned for discrete mode.
Let's take the example of self-driving cars, at each X ms$X$ms (e.g X=1$X=1$), you'll be computing the state of the car which are your input features (e.g direction, orientation, rotation, distance to the pavement, relative position on the lane.., etc.) and take a decision of the action to take as in discrete mode. The approach is the same in other use cases, like playing games, walking robot..robots, and so on.
Note (continuous action space):
If you have continuous actions, then in almost all use cases the best approach is to discretisediscretize your actions. I can't think of an example where discretisingdiscretizing your actions will lead to a considerable deficiency.