Protecting the pawn and preparing b5-b4, but there's a better move with the same idea.
1. a1b3 Interestingly, in this starting position the weakest square is a2-a7, which has no protection. In the conventional chess position the weakest point is f2-f7, which at least has the protection of the king. The move 1.f4 therefore creates an immediate threat right after the first move. 1.Nb3 brings the knight into play and protects the weakness on a2. This move was played in all the games of the round. 1... d8c6 There are many alternatives. The opening theory in each position of Freestyle allows for many hours of discoveries. 2. e2e4  2... e7e5  3. f2f3  3... f7f6  4. d1c3  4... a8b6  5. c1b1 The moves by both players up to this point were very logical. Firouzja, needing a win, decides to spice things up a little. 5... f8b4  6. e1g3  6... e8e7  7. c3b5 This looks like a strange move to me, but the computer doesn't consider it wrong. 7... a7a6  8. b5c3 By going up and down with the knight, the b6-square has been weakened and I think this was Keymer's plan. He now can consider capturing on b6 in a better version, but Firouzja also has some ideas. 8... b4c3  9. d2c3  9... b6a4  10. g1e3  10... b7b5 An excellent move. More than going for b5-b4, which can be met by c3-c4, Firouzja secures the c4-square for the bishop. 11. f3f4  11... g8c4  12. g3g4 This allows Black to capture on f4. 12... e5f4  13. g4f4  13... c6e5 with firm control of the e5-square and a potential attack on the queenside, Black is clearly better. 14. e3d4  14... c8h8 As is usually the case, after both sides have castled the position on the board no longer looks strange to the standards we're used to. Black has a clear plan of attack on the queenside, while it's not clear what White should do. 15. f4g3 The best plan is to reduce Black's attacking potential. The queen is helpful for defending the king along the third rank. 15... c4f7 An interesting move, vacating the c4-square for the knight. 16. d4e5 It's necessary to capture the knight before it goes to c4. 16... f6e5  17. f1e2  17... c7c5 Black starts a pawn advance on the queenside. White's position is very unpleasant. 18. d1f1 The old problem of which rook to move. Keymer probably considered it would be useful to give the king some space in case it needs to run from the attack, but the rook was useful on d1. 18... c5c4  19. b3d2 