Understanding the Tarrasch defence to the Queen's gambit I do not understand the diagrammed position. Do you?

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[title "A. Grischuk vs. V. Kramnik, FIDE Candidates 2018"]
[startply "8"]

1. c4 e6 2. Nc3 d5 3. d4 Nf6 4. Nf3 c5 5. e3 dxc4 6. Bxc4 a6 7. Bb3 b5 8. e4 cxd4 9. Nxd4 Bb7 10. e5 Ne4 11. O-O Nxc3 12. bxc3 Nc6 13. a4 Bc5 14. axb5 axb5 15. Rxa8 Bxa8 16. Nxb5 O-O 17. Qxd8 Rxd8 18. Bf4 h6 19. h4 Ne7 20. Rd1 Rb8 21. Bc4 Bc6 22. Nd4 Be4 23. h5 Nd5 24. Bxd5 Bxd5 25. Be3 Rc8 26. Re1 Bc4 27. Nf3 Ba3 28. Bd4 Bd3 29. Nd2 Bb2 30. Nf1 Rc4 31. Re3 Bf5 32. Ng3 Bh7 33. Kh2 Bc1 34. Re2 Bf4 35. Kh3 Ra4 36. Kg4 Bc1 37. Kf3 Bd3 38. Re1 Bb2 39. Ke3 Bc2 40. Kd2 Bb3 41. Rb1 Ra2 42. Kd3 Ba4 43. Ne4 Ba3 44. Ke3 Bc2 45. Re1 Bb3 46. g4 Bd5 47. Kd3 Be7 48. Rb1 Ra8 49. f4 Bh4 50. Rh1 Bd8 51. Rb1 Bc7 52. Ke3 Kh7 53. Nd6 Kg8 54. Rb5 Bc6 55. Rb4 Bd8 56. Bb6 Bh4 57. Bd4 Bg2 58. Rb2 Bc6 59. Rb6 Bg2 60. Rb2 Bc6 61. Rb3 Bg3 62. c4 Bh2 63. f5 exf5 64. Nxf5 Kh7 65. Bb2 Re8 66. Nd6 Re7 67. Rb8 f6 68. Rc8 Bh1 69. Nf5 Rb7 70. exf6 Rb3+ 71. Bc3 gxf6 72. Kd2 Bf4+ 73. Kc2 Rb8 74. Rxb8 Bxb8 75. Ne7 Be4+ 76. Kd2 Bf4+ 77. Ke2 Kg7 78. Nf5+ Kf7 79. Bd2 Be5 80. Bxh6 Ke6 81. Be3 Bxf5 82. gxf5+ Kxf5 83. h6 Kg6 84. c5 f5 85. Kf3 Kf7 86. Bf4 Bd4 87. c6 Ke7 88. c7 Kd7 89. h7 Kc8 90. Ke2 Kd7 91. Kd3 1-0


What is the idea behind 4... c5? Why form a hostile bloc of two black against two white pawns on the c and d files? Once the bloc is formed, how does one decide whether


to capture toward the c file (as Black soon does),
to capture toward the d file (which, inscrutably, neither player wishes to do), or
not to capture at all?


This kind of position is opaque to me. I do not understand it. I am aware of no principles by which one can treat it. I perceive no relevant pattern in a bloc of four pawns. Please advise.

I would gladly read an answer at any level but would find most useful an answer pitched at about Elo 1350 or USCF class C.

(Source for the game: Chess.com.)
What is the idea behind 4... c5? 


Basically the same ideas as behind playing c4 in the Queen's gambit: 


challenging the center,
potentially exchanging the c-pawn for a valuable central pawn, 
opening the c file for a rook, 
giving the knight a square on c6 (which is often a better square for the knight if the pawn has moved to c5 already)
potentially threatening to leave white with an isolated queen pawn (see below)



  Why form a hostile bloc of two black against two white pawns on the c and d files?


Why not? "Hostility" is not something you should be afraid of when playing chess.


  Once the bloc is formed, how does one decide whether ...


There is no strict rule here and various captures and no-captures have been tried and tested by good players. A typical motive you need to be aware of in these positions is the isolated queen pawn (IQP), i.e. a player might be left with a pawn on the d-file and without any pawns on the c and e files which could support it. 

An IQP leads to somewhat asymmetric play, with one player trying to block and put pressure on this potentially weak pawn, while the other player (the one with the IQP) is trying to take advantage of his better piece play that is a result of the pawn structure and also has ideas related to advancing the IQP.

If there is no IQP on either side, the positions resulting from this opening are often very symmetric, potentially boring, and it is difficult to find a good plan in them.

So I'd say the decision on how to capture is basically determined by what kind of game you want:


with an IQP; asymmetric, unbalanced game with clear plans for both sides
without an IQP; symmetric, equal position, with unclear plans