Austin Shenton, of the Tampa Bay Rays, prepares for a pinch-hit appearance against the Boston Red Sox.
Pinch Hitters
A pinch hitter is a player, previously not in the lineup, who is called upon to hit in place of another hitter. You will typically see pinch hitters used later in the game, either to counteract specific relief pitchers that are brought in, or to attempt a specific strategy—but also any time one of the previous players has been injured or otherwise needs removing from the game.
A pinch-hitter does not become official until they have been “announced into the game,” which means being recognized as a replacement by the home-plate umpire, and then officially, yes, announced by the PA system. Once this happens, the hitter who had been in that lineup spot is removed from the game.
A lot of the strategy of pinch hitting revolves around reacting to relief pitchers, especially when it comes to pitcher handedness1. As such, you might see a team send a pinch hitter out on deck, but pull that player back if the other team makes a pitching change. Alternatively, you may see a team send out a pinch-hitter, let the other team make a pitching change, then replace that pinch-hitter with another pinch-hitter. This type of strategy burns the first pinch-hitter without them ever seeing a pitch, so you won't see it quite as often as a simple pinch-hitter scenario.
Depending on the game state, if a pinch hitter gets on base, they may immediately be replaced by a pinch runner, who takes the place of the pinch-hitter on the basepaths. Either way, once the inning ends, the offensive team will go back out on defense (unless the game ends, of course); at this point, the pinch-hitter will need to take the field2; either the pinch-hitter will slot into the defensive position that the original batter was playing, or the team will shift another player into that position, and the pinch-hitter will go out to replace them. So, say a team pinch-hits for their shortstop. The next time they go out on defense, the pinch-hitter can go play shortstop, or another player— say, the second baseman, can slide over to short. In this situation, the pinch-hitter would go play second. Alternatively, the pinch-hitter could be replaced by an entirely new player on defense. However, all that happens later— what matters right now is that the pinch hitter approaches the plate, ready to hit.