Do you get to choose your hand, or is the highest hand chosen automatically?
In other words, do you choose how to play your hand in the end, or is the most valuable combination of cards automatically and involuntarily chosen?


What a player claims to have, is irrelevant. The one and only thing that matters in deciding a player's hand is what cards that player may use. The one and only claim of a hand that matters is putting the cards face-up on the table.

This is often known as cards speak (i.e. "the cards speak for themselves").

Allowing players to declare a hand which is suboptimal would open the game to all manner of collusion and nonsensicality.


  Assuming an opponent has a hand that is better than a pair of kings but worse than a royal flush, do you win because your hand can form a royal flush, or do you lose because you didn't notice the royal flush and tried to go with pair of kings?


The player with the royal flush has a royal flush. No other player can have an equal hand, and no other player can have a better hand. Therefore, the player with the royal flush (holding KK) wins the hand and any pots they are part of.