Quote: "so that means, we can modify and compile it. then replace it in the FPSC root directory, then build our level and have our changes in our game."
Yes, you can do exactly that.
There are other possibilities too - for example you could create a new particle entity, which the modified game exe then handles.
There's no getting away from it - the game engine source is very large and quite complex. Don't expect a line-by-line break down or anything
But if you're a competent programmer it's an option certainly. And yes people could then share their modified exes with the rest of the community.
People don't quit playing because they grow old.
They grow old because they quit playing.