Petrosian x Smyslov 1951 - Help

1. d4 d5 2. c4 dxc4 3. Nf3 Nf6 4. Nc3 c6 5. e4 b5 6. e5 Nd5
7. a4 e6 8. axb5 Nxc3 9. bxc3 cxb5 10. Ng5 Bb7 11. Qh5 g6
12. Qg4 Be7 13. Be2 Nd7 14. h4 h5 15. Qg3 Nb6 16. O-O a5
17. d5 Nxd5 18. Rd1 Qc7 19. Ne4 O-O-O 20. Bg5 Bxg5 21. Qxg5 a4
22. Qg3 f5 23. Nd6+ Rxd6 24. exd6 f4 25. Qxg6 Qxd6 26. Bf3 Bc6
27. Re1 Re8 28. Bxd5 Qxd5 29. Rad1 Qf5 30. Qxf5 exf5 31. Rxe8+
Bxe8 32. f3 Kc7 33. Kf2 Kb6 34. Ke2 Ka5 35. Rb1 a3 36. Kd2 b4
37. cxb4+ Ka4 38. Kc3 a2 39. Ra1 Ka3 40. Kxc4 Kb2 41. Re1 a1=Q
42. Rxa1 Kxa1 43. b5 Bd7 44. b6 Bc8 45. Kd4 Kb2 46. Ke5 Kc3
47. Kxf4 Kd4 48. Kg5 Ke5 49. Kxh5 Kf6 50. g4 Bb7 51. Kh6 1-0

- Why did Petrosian play 20. Bg5? Just to trade bishops?

- After 25. Qxg6 computer says white is much better, but why? Pawn structure?

- If 28. Bxd5 is suppose to go after a winning ending for white (according to what I read), than why it took one move for Petrosian to play it?

hey ghost_of_pushwood, help me with this other one: Zsofia Polgar vs Vladimir Kramnik
Wch U18 (1991), Guarapuava BRA, rd 6, Jul-?? - https://www.chessgames.com/perl/chessgame?gid=1085646

[Event "Wch U18"]
[Site "Guarapuava BRA"]
[Date "1991.07.??"]
[EventDate "?"]
[Round "6"]
[Result "0-1"]
[White "Zsofia Polgar"]
[Black "Vladimir Kramnik"]
[ECO "B33"]
[WhiteElo "?"]
[BlackElo "?"]
[PlyCount "90"]

1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5 6. Ndb5
d6 7. Bg5 a6 8. Na3 b5 9. Bxf6 gxf6 10. Nd5 f5 11. Bd3 Be6
12. c3 Bg7 13. O-O Bxd5 14. exd5 Ne7 15. Qh5 e4 16. Bc2 Qa5
17. Rae1 Ra7 18. Kh1 Bxc3 19. bxc3 Qxa3 20. Bb3 Qb2 21. f3 Rg8
22. g3 f4 23. fxe4 fxg3 24. Rxf7 Kd8 25. Qh6 Qxc3 26. Rf8+
Rxf8 27. Qxf8+ Kd7 28. Rf1 Qe3 29. Qf4 Qxf4 30. Rxf4 gxh2
31. Bd1 Kc7 32. Rf7 Kb6 33. Rxh7 Rc7 34. Kxh2 Nxd5 35. Rxc7
Nxc7 36. Kg3 Kc5 37. Kf4 Kd4 38. Kf5 b4 39. Ba4 a5 40. Bc6 Kc5
41. Bd7 Nb5 42. e5 dxe5 43. Kxe5 Nc3 44. Be6 Nxa2 45. Bxa2 a4
0-1

What would you say is to be taken from this game? Seems like a more tactical one, but still feels like there is something to be learned here, can't tell what exactly. Maybe the ending?

Bg5 is to trade the bishops - the e7 one was defending d6 nicely (stopping white doing Nd6+)

Qxg6 is better for white because of the material primarily. @ghost_of_pushwood answered your last question nicely.

20. Bg5 has a couple purposes. 

- Note that white's bishop on c1 has no prospects of doing anything, aside from trading itself off for black's bishop. On c1, it's just in the way of white's pieces. Moving the bishop helps connect white's rooks.

- Once white gets rid of black's dark squared bishop, black has trouble covering weaknesses on the dark squares. The most obvious weakness is d6, where white's knight gets access to a tremendous outpost. Black has no good way to get rid of that knight without giving up a rook for it - which he does, because the knight is so strong, and black's rooks can't accomplish much anyway while the white knight is on d6. A knight on that kind of an outpost is worth a rook.

20. Bg5 offers a trade of white's bad bishop for Black's good bishop.

Thanks everyone. But I realized something quit important today, apart  from not trying to play fast games where I'm barely able to look after my pawns. I need to look for more basic theory and theory that is easier to apply on my correspondence games.

r2qnr2/1p1b2bk/n2p3p/p1pPpp2/2P4N/PPNBB1PP/3Q1P2/1R2R1K1 b - - 0 1 - Why is this position such a strong attack (+2) according to the computer?