Regarding the Dragon

Well, I did learn that the dark-squared bishop of Black is vital in the Dragon variation of Sicilian Defense. Therefore, as White, I should trade bishops.

However, when I played as White: 7.Qd2?! O-O 8.Bh6?!, the computer said that were inaccurate moves. Could someone explain it?

What do you mean computer said those moves were inaccurate? As in illeagal moves, or that they are not dragon defense? Or that it is a "bad move"?

Also which computer are you referring to?

The computer analysis of chess.com. What I mean is that the computer refered 8.Bh6 as a bad move, and I dont know why.

after 7.Qd2 I believe black has 7... Ng4, forcing the trade of his knight for your dark sq bishop.

usually f3 is played to prevent Ng4, and also give strength for future g4 and h4 pushes

7.Qd2 is inaccurate because of 7..Ng4 which will get rid of your DSB for the knight (indirectly assuring preservation of blacks g7 bishop without trouble).

I dont really know about 8.Bh6 being inaccurate, but maybe black can play 8..Bxh6 9.Qxh6 Qb6 10.Nb3 Ng4 which looks a little annoying for white.

In any case, the computers evaluation of strategic ideas such as trading off blacks dark square bishop generally are not very good. It's analysis should be mostly used to check for blunders in your game.

White wants to trade the darksquared bishops. But he also wants to develop! There are tons of things that both players wants, only doing 1 thing (trading the "dragon head") is usually a bad thing, since you forget the rest (develop your pieces, get your king to safety, start an attack).

Im not sure about a concrete line, but black also played some slight inaccuaries. Obviously, 7... Ng4 was just good for black, but instead of castling, Nc6 is very logical, because the knight is well-placed there, and it prevents some lines (such as Bh6, because of Bxh6 and Nxd4