The Catalan begins after 1. d4 Nf6 2. c4 e6 3. g6
The first thing you should notice is that we are doing is a fianchetto of the light-squared bishop. So likely, our next move is Bg2, even if they play d5.
If they take on c4, we play Nf3. If they play b5, saving that pawn, Ne5, attacks the rook and the only move to defend is Nd5. We will castle, and we can easily get the center with e5 after they move. If they play Bb4+, we will block with Bd2. If they trade, recapture with the b1-knight, if instead they play Nd5, trade bishops and castle. If they don't play Bb4+ you attack c4.
Okay, dxc4 was inaccurate, the better move is Bb4+. We block with Nd2. If they trade, recapture with the bishop, and if they take on c4, Qa4+ forks the pawn back. If they castle, play Nf3. Now if they take on c4, we play a3, if they trade, recapture with the bishop and play e3 if Ne5 isn't possible. If they don't trade, with Ba5, castle.
If other than all that they play Be7, we play Nf3, they castle and we castle, if they take here, we play Qc2, and after b5 undermine with a4. If instead of b5 they play a6, play a4 then take.
That's all to the Catalan opening, it's more of a setup based opening and in my opinion, is kinda boring. You can(should) only play it as white with d4. Good luck and have fun with this opening.
