IM Samuel L. Shankland played in the true character of a modern day chess master. He isolated all black's materials to one quadrant of the board along with his opponent's castled king, posted his queen at the center, and his two bishops doing their tasks in a well-coordinated fashion.
The endgame is I believe something he had planned from the beginning. In my own 1900 or less rating level assessment, the move 29.Bf1 was a subtle winning move preserving his light-squared bishop to assist later on his dark-squared bishop along the long diagonal with his queen. The position after 38.Bb2 was the realization of his overall strategy. See image below.
His preparations must be rigid, similar to those described in the article posted here three weeks ago about modern chess. He moved his pieces with software precision, very calculating in every way, never taking unnecessary risks - a winning character that I don't have - the reason why I always say that chess is a game of patience.
GM Christiansen, playing in a classical and a hypermodern fashion, tried to storm the barricades once more using his knight and queen to attack but Shankland built a strong fortification.
His two bishops were pointed like canons along the cental diagonals together with his queen towards Christiansen's king pinning it along with the g7 pawn. Christiansen was caught flatfooted when Shankland unleashed one of his canons taking h6 shattering the former champion's own barricades.
The wall started to crumble losing his pawns to the methodical and patient moves of the younger master and forcing the three-time US Chess Champion to finally surrender.
Perfect game from Shankland while Christiansen had three errors such as 44...Bf5, 47...Bf5, and finally 57...Qb8 according to ICC's Dasher.