Ulhumbrus: 6...Nxc3 moves a knight for a third time to exchange it for a knight moved once, making a loss of two tempi for development and it strengthens White's centre as well. This does not mean by itself that it has to be wrong, but if a player is going to make such a triple concession to the opponent he should have adequate justification for making it.
10...Bxf3 doubles White's f pawn but it concedes the bishop pair and strengthens White's centre. Once again this by itself does not mean that it must be wrong, but if you give the opponent so much, you should know why.
11 d5 gains some space but it obstructs White's king's bishop, releases control of e5 and removes an obstruction from the long diagonal for Black's king's bishop.
12...a6 moves a pawn in the opening, that is, before Black has completed his development. 12...Nd7 gets the knight out. Black plans to advance his b pawn to b5 but the b pawn becomes on b5 a target instead of a weapon.
After 17 Ke2 White has an extra pawn, the bishop pair, more space and central pawn superiority.
He has however against all these assets one main liability: his king has not reached safety yet.
This can be a serious liability indeed but only if Black can make it count. The result of the game will hang on whether Black can make this liability count.
That is not so easy because White has more space.
To make count White's king's lack of safety Black has to open lines against White's king.
To open lines against White's king Black' pawns have to make contact with White's pawns. How may Black arrange this?
One possible plan is to advance Black's e pawn to e6 so that Black's e pawn makes contact with White's d pawn. Another possible plan is to advance Black's g pawn to g4 so that Black's g pawn makes contact with White's f pawn
17...Ra7 makes no attempt to begin contact between Black's pawns and White's.
With 18 f4 White advances a pawn. One reason to not play a pawn advance such as 18 f4 is that if White advances a pawn he reduces the distance Black's pawns have to travel in order to make contact with his pawns. Now Black's g pawn needs only to advance to g5 to make contact with White's f pawn instead of to g4. Besides, Black's e pawn can make contact with White's f4 pawn by ...e5.
With 19...Nf6 Caruana makes no present attempt to initiate contact between White's f4 pawn and Black's e or g pawn. Only much later at move 33 does he do it by 33...g5. By then it is too late because White's king has managed to reach relative safety on the king side.
21 Bxf6 relinquishes the bishop pair. Although the bishop may seem inactive and more so after Bh4 and Bg3 White may be able to bring into play after his king has reached h1 and greater safety.
Caruana lost mainly because he did not try to make count White's main liability - an unsafe king - by opening lines against White's king by initiating contact between his pawns and White's until after White's king reached relative safety, when it was too late