If i want to learn the catalan do i have to know the QGA and the slav

I'm planning to learn the Catalan but in all the books i've looked at don't cover the QGA aswell as the slav. Should i buy a book about the QGA from the white perspective and do the same for the slav ?

The Pseudo-Catalan move order has the upside of completely bypassing the QGA and Slav variations. I'd recommend it over any other Catalan move order.

This move order is bad because black simply plays Bf5 and has a positional advantage. It offers white almost nothing.

There are many move orders to get to the catalan. Each of them have their own thing, since the catalan isn't one of those easily forced openings. I use the 1.Nf3 move order, to avoid ideas with Bb4+. This means I usually have to play an english style position, and rarely get a catalan. Trying to go with 1.Nf3 and then 2.c4 and 3.g3 means you need to study lines involving ...dxc4.

You do not need to buy a book on the QGA. Watch a couple of videos, learn some new ideas. That is more important.

Doesn't White just play 4. c4 here? And then after 4... c6 5. Nc3 White should be fine. It won't be a Catalan but White does fine in the Slav. Besides, 3... Bf5 isn't the most common move, it's played roughly 11% of the time here.

White plays c4, black plays c6, etc, play goes into a standard closed Catalan… except black has his LSB out. This makes a significant difference. White would prefer his pawn to be on d3