Sicilian d6 e5 structures.

I want help understanding a sicilian structure that comes out of the najdorf or schevenigen sicilian.

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k7/pp3ppp/3p4/4p3/4PP2/8/PPP3PP/K7 w - - 0 1

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https://lichess.org/analysis/standard/k7/pp3ppp/3p4/4p3/4PP2/8/PPP3PP/K7_w_-_-_0_1
The whole point of the Najdorf is to play ...e5 without being hindered by Ndb5. Black accepts a hole at d5, but white cannot profit from it. Black often captures ...exf4 so as to install a knight Ne5.

Okay, there we go. So I want to know in this position how to continue as black as white is challenging your centre. Pushing d5 leaves e5 undefended. The most common move is to take on f4, but why is this the case? Doesn't this go against the Sicilian? You both have an equal amount of central pawns but white simply has more space.

Such as in the mainline schevenigen classical

@tpr I didnt see your reply, but I didn't think of playing Ne5, Both players get an outpost in the centre, interesting.

In that position black pushes 12...d5 later. Example: