Wiki re: Catalan Opening:

Below is a link for the wiki re: the Catalan:

https://en.wikipedia.org/wiki/Catalan_Opening

IMHO the rather brief wiki doesn't cover this opening all that well, only mentioning two general (rather than specific) approaches by Black, namely to open or keep closed the position based on whether ...dxc4 is played (in particular, there are several major systems as classified in ECO, such as with or without an early ...Be7 being included).

What also may be of interest to many is the subject of specific move orders to get to (or possibly avoid) the Catalan. In the case of 1.d4 Nf6 2.c4 e6 3.g3 nowadays Black often chooses 3...c5, avoiding the Catalan with the aim of obtaining a Symmetrical English line in the event of 4.Nf3, or else a g3-Benoni line in the event of 4.d5 - the latter is not thought a particularly potent way to play vs. the Benoni. As an aside, I'd note if Black is e.g. a Benoni player he might consider 1...c5 since 2.d5 is objectively best, but then White doesn't get to transpose to a Symmetrical English if he otherwise had wished to avoid e.g. the Benoni (or the Schmid Benoni [with ...g6 and ...d6], if he chooses 3.Nc3 instead of 3.c4 after 2...Nf6). Back to the topic of the Catalan, a more common way to get the opening these days would be 1.d4 d5 2.c4 e6 3.Nf3 Nf6 4.g3.

Considering the most respectable flank opening moves by White at turn one (namely Nf3 or c4), 1.Nf3 (discourages 1...e5 but commits the Kt) could be met by 1...d5 if Black actually likes to face the Catalan (most Blacks nowadays seem to prefer to avoid it if convenient); he could answer 2.g3 by 2...e6 (many Blacks may prefer to go for a g3-Slav line, or even to aim for a Neo-Grunfeld by 2...g6, instead, for example), but then White can deprive him of some options against the Catalan by continuing 3.Bg2 Nf6 4.0-0 and now 4...Be7, 4...Bd6 or 4...c6 would all commit Black to some degree in case of transposition to a Catalan, by c2-c4 and d2-d4, and Black may not like any of that. Black could try 4...c5, but then 5.c4 can lead eventually to a QG Tarrasch (something many people prefer not to use these days). He also could try 4...b5!?, stopping c2-c4, to avoid White's attempt to cramp his style a bit - however he pays a price in a sense if White now plays 5.d3, aiming for a King's Indian Attack where Black is committed to an unusual but playable line where he'd intend to play ...Bb7, ...c5, ....Nc6 and ...Be7 - so Black has lost options in the KIA instead. As an aside, I'd note that after 1.Nf3 d5 2.c4 Black has the extra options of 2...dxc4 and 2...d4, while if 2.d4 Black still has a lot of options (some more repectable than others) besides aiming for a QGD or Slav setup - these are some ways White can feel 'punished' for playing 1.Nf3, say instead of 1.c4 (which allows 1...e5 though, most notably).

Back to the topic of the Catalan. In case of 1.c4 if Black actually likes playing against the Catalan, he could play 1...e6, and after 2.Nf3 d5 (a position that can be reached by 1.Nf3 d5 2.c4 if Black now commits himself with 2...e6 - a point some might see as a possible defect of 1.c4 e6 [1...c6 has a similar story]), White could try to get him to give up some options against the Catalan by playing 3.g3 with the idea of delaying d2-d4 for a bit. However, here Black has more than one way to thwart this aim, if he wishes to - e.g. after 3...Nf6 (even 3...g6 gets respect in ECO[!]) 4.Bg2, Black can play 4...dxc4, and then play doesn't have to eventually transpose to any Open Catalan variations, by any means, though the details are complex.