At the beginning of a game high level players learned a set of good moves and good answers by their oponent and the answer to that and so on. That can be 5 moves or 25 moves that they just learned, depending on the strength of the players and the strength of the opening. An opening that is strong will be played more often and it is worth it to memorize more of it.
In the middle game, unless players are forcing specific moves (“If they dont move their queen to this square, I can win with a mate”) it is hard or even impossible to calculate more than 5 moves ahead. Even with some pieces off the board there could be hundreds of ways to make 5 non-idiotic moves each. Then good chess players might take a lot of time, consider different moves, imagine what can happen next, how that would change the game and so on. It helps a lot to have good memory, a lot of games played and remember certain patterns, it makes calculating moves much easier.
In the endgame with less pieces on the board it becomes easier to calculate and therefore the number of precalculated moves can increase again. With very few pieces some endgames are completely solved and pro players might know some of them completely. Especially those that appear often after their prefered openings.
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