Sicilian game
2.Bc4 is usually not as good as 2.Nf3 or 2.Nc3 or other 2nd moves.
2...g6 is OK, but Black can play almost anything here.
3.Nf3 looks normal. Some players try to play 3.Qf3, hoping for 4.Qxf7+. Sometimes, that can backfire. 3.Qf3 e6 4.Qf4 Nc6 5.d3 Bh6 wins a piece SSJ-Wall, Internet 2001.
Not sure what 5.a3 does. I have seen 5.Nc3 or 5. c3 or 5.d3 in my games. Black played 5...Nf6, but can also play ...d6 or ...e6.
7.c3 seems to be new. My database has 7.Nc3 a6 8.Be3 d6 9.h3 b5 10.Ba2 and White eventually won, Talmazan-Ghiugan, Romania 1999.
13...d5?! looks like it drops a pawn after 14.Bxc5. Perhaps 13...a5 and attack and expand on the queenside or try 13...Ne7, then 14...d5.
16.Nb3?! does nothing. Perhaps 15.Nhf3 to get the knight on the rim more active.
18.Nc5? looks bad. The better move is 18.Nd4. After 18.Nc5, Black misses 18...Ncxe5, winning a pawn (threatening 19...Nxc5). If White playes 19.Nxd7, then 19...Nxd7. If 19.Nxe6, then 19...fxe6 20.fxe5 Nxe5 and Black is up a pawn.