It is always the best five cards that make a five card poker hand that determines the winner loser or split. Holdem is played with seven cards, and the two cards that are not part of the best five card hand count for absolutely nothing in determining if a pot is won lost or split.
No, having 5 cards in blackjack does not automatically result in a win. In fact, the objective of blackjack is to have a hand value as close to 21 as possible without exceeding it while also having a higher hand value than the dealer. The number of cards you have in your hand doesn't guarantee a win.