How do graphic modifications of games work?

Question

Graphic arts

I don't know if you have installed the ENB graphics mod for GTA, but it changes the graphics dramatically.
I wonder how the developers of such modifications achieve this?
Often such modifications - and not only such ones - have a d3d9 file, which, I think, connects everything that changes in the game.
But it's still unclear how they calculate what needs to be added to this file, how they keep track of what needs to be added to change the value, etc.
If anyone knows, share, if not difficult. I'm interested in this topic.

Answer 1

To do them, you need to be able to conduct such research here. Accordingly, you need to know by heart the graphic API on which the game is written. We have already talked about reverse engineering.

Answer 2

I don’t know what ENB and GTA are, but d3d9 is DirectX 9 (now version 12)