Help me deeper understand Giuoco Pianissimo opening. What are key positional ideas for white and black in the middlegame? Which squares are most important? Is it good to exchange bishops on e3 and capturing with a pawn or should i avoid it? Should i keep white square bishop at almost "all cost"? Should white always avoid pin on f3 knight?
- one key idea for white i know is to bring queenside knight to g3 to fight for f5. 

-for white its almost always kingside attack, playing f2-f4 in some positions

- pawn break at d4 at some point

 u ask too much...

There are some books or dvd for that...

there is a lot that could be said about the giuoco pianissimo in general.

first of all...yes it is generally fine to play be3 and allow the capture. the opening of the f file tends to be good for white.

No, you do not need to keep your light square bishop at all costs. especially if you are unable to manage the d4 break, this bishop winds up being weak. You don't need to go completely out of your way to avoid the pin on the f3 knight, but you should be very vigilant about it. This can create problems for white potentially. Usually a maneuver like c3, h3, nd2-f1-g3 is enough to break the pin.

F5 is another important square you forgot to mention and if you can get a knight to f5 you can create kingside pressure. But be vigilant about moves like Nh4 nxe4 discovering the queen attack on the knight

very often white can create pressure on the queenside rather than the kingside...or in addition to it. this can be especially true in lines where you decide to play for b4 (a key move) and/or nd2-nc4 and/or Qb3. So no, in a refined Italian game you are not 'almost always' playing for a kingside attack.

Because it tends to be a rather slow and steady opening without either side being forced to do that much, it is difficult to give a concise explanation of the strategy...because so much is possible for both sides. That is why I would not recommend such an opening for newer players...I think they can get themselves lost in all of the playable options.

The trouble is, there isn't such a thing as the Italian game, because e4, Nf3 and Bc4 etc can lead to passive or wild positions and anything in between.

Oh, you state Italian Game in the title and Pianissimo in the text. I thought the main idea in the Quiet Game was to develop the pieces, hope the opponent doesn't try anything violent, and ask for a draw.

Possibly of interest:
Winning with the Slow (but Venomous!) Italian by Karsten Müller and Georgios Souleidis
https://www.newinchess.com/media/wysiwyg/product_pdf/9037.pdf

hi

Well, one thing is for sure, You are going to avoid trading your light squared bishop for opponent's knight in general. 

For about a century, it was generally understood that when Black played Be6, opposing White's light-squared bishop, it was usually a bad idea for White to play Bxe6. While Black got doubled e-pawns, he also got an open f-file to attack and a central pawn majority.  This is certainly how I learned it, along with everyone else. But in the last few years, Carlsen, So, and Caruana, among the top GMs have started playing Bxe6 as White and taking finding ways to blunt Black's attacking chances and to exploit the ungainly doubled pawns. In it's own quiet way, this is a revolutionary change in the way the top players handle these positions. There must be a book somewhere that explains this change in thinking, but I haven't seen it.

It isn't a change so much as a reversion to a quieter style that became prevalent in the early 1900s and probably met its peak with players like Edward Lasker and Capablanca.

The pianisimmo is not just about getting a quick draw. Do you know anything about positional play?

No.

Well, if I wanted to play positionally to win, I wouldn't be using 1. e4 and playing the Italian Quiet game. I believe that white makes too many concessions and by doing so, reduces the chances of a win.

This is an example of me trying to play positionally, although I normally try to be more tactical.

?? The big reason why top players play this way nowadays is to preserve pieces on the board, thereby keeping some fight in the position so that they can play for the win. White does not commit to any plan until later in the middlegame, thereby maintaining their flexibility.

I play 1. d4 to keep the pieces on.

Makes more sense maybe.

There's one or two defences where black can get a couple of pieces off but it gives white a positional advantage.

@16
"The big reason why top players play this way nowadays is to preserve pieces on the board, thereby keeping some fight in the position so that they can play for the win."
++ Yes, indeed. This is how Nepo played in a must win situation:
https://www.chessgames.com/perl/chessgame?gid=2136417

He refrained from the Ruy Lopez and chose the Giuoco Pianissimo.
He refrained from the sharp 4 Ng5 or 4 d4 and chose the tame 4 d3.

He also showed that he wasn't a worthy contender, so I doubt we can read much into his choices.

@16
Here is a more recent top game where white prevails:
https://www.chessgames.com/perl/chessgame?gid=2292579