Poker rules state that when the final bet is called, players should show their hands in order, starting with the last raiser and continuing to the last caller.
However, players whose hands can't beat the best hand often choose to muck their cards instead of showing them.
Yes. A player who calls the final bet has effectively paid to see the opponent's cards.
On PokerStars, you can request the hand history to review the cards that were called to showdown.
Mucked hole cards at showdown are consistent with accepted poker rules.
Players who call the final bet can request to see the relevant cards, but folded hands remain hidden.