The first equation intuitively means that the expected value of a given state and action can be determined by the reward at a given time-step plus the expected maximum return at the next time-step. It doesn’t need to determine all Q values for all actions, only the one which is the maximum, since the algorithm acts optimistically in regards to its expected reward over time.
The loss function is a way of determining how far the current Q estimation is from the Q estimation if we take into account one future timestep’s expected reward. Since an agent can only ever take a single action, Q-target is calculated using both the action taken and the maximum-valued future action. We also only want to update the Q value for the action we have taken, since that is the only action which we know what state will follow from it.
I hope this helped clear things up.