I don't get stockfish

Why is 37.h6 a blunder??

I think it's because of the manoeuvre Ke7 Kf7 Kg8 (eyeing the bishop but leaving a path for the knight) Nf7, which threatens to take the h6 pawn. The threat is to deflect the bishop from the b2 and a1 squares, allowing black to promote using their queen-side pawn advantage. h6 moves the pawn away from the king, so the king does not have time to climb the board and defend that square. If it tries to, then the knight can hop around and prevent the king from further progress resulting in a draw (eg 37. ...Ke7 38. Kf2 Kf7 39.Kg3 Ne4+ 40. Kh4 Nf6 41. Kg5 Nh7+). The only options for white are to move the bishop to the other side of the board to stop promotion and let black win the h pawn, or walk the king over the to queen-side pawns and take them, once again allowing black to win the h pawn with Kh7 and Nxh6 (defended by the king). Both of these result in an equal position.

The best move a3 is meant to disable black from playing b4, threatening a promotion on the b1 square which cannot be controlled by the bishop. If black instead threatens a promotion on the a1 square (with b4 followed by a3 and a2), white's bishop can monitor both the promotion square and the mess in the top right hand corner. White can reach a position with a bishop on a1 and pawns on f5 and h6, a position that black cannot break. The knight is confined to the f6, g5 and h7 squares to prevent the h pawn from promoting, and the king must stay by the knight to prevent the bishop from taking it. In that position, the white king is free to walk over to blacks queen-side pawns and win them.