Well it’s time now to look at things from the perspective of black and the well known Sicilian Defence which is the most popular response from black. Instead of responding by copying black opens up his own space on the queenside.
This is the reason why d4 is now actually preferred for white because of how strong this response is from black. White usually continues with Nf3 to try and attack and control the centre and prepare d4 which will then be defended by the knight. Notice that black now has two centre pawns versus one for white which is what makes the Sicilian so strong. But white gains a lead in development to offset this.
White then responds with d6 defending the c5 pawn and stopping white from pushing to c5. But the bishop on f8 is now blocked in for black so will need a further move to develop this in the future such as e6.
The main line is d4 from white which black captures and white recaptures with the knight which is known as the open version of the sicilian. Black moves Nf6 attacking the e4 pawn and white responds with Nc3 defending.
Pawn to a6 is the main line and known as the Najdorf variation. Nc6 continues the Open version of the Sicilian and g6 is known as the dragon and we’ll look at some of those variations next.
Here’s a great video from the chess website going through the Sicilian Defence.