C++ :: SDL Library - Access To Pixel Data

I am using the SDL library, and came across a way to access pixel data.

//A pointer to a surface structure

SDL_Surface *w = SDL_SetVideoMode(800,400,8,SDL_SWSURFACE);

//Casting a void pointer pointing to an array of memory locations(the pixels), to a pointer to an array of Uint8's.
//SDL_MapRGB() Returns an Uint32 containing the color 0,0,255,(0/255, i think it returns the transparency too, or else it would rather return a Uint24,not sure).

Then modifying the memory location, in this case the pixel at the middle of the window.

static_cast<Uint8*>(w->pixels)[(200*w->pitch)+400]=SDL_MapRGB(w->format,0,0,255);

What i seem to not understand is that, i cast pixels to Uint8*, which means I now have a pointer to an array of 8bit numbers, but SDL_MapRGB seemingly returns an Uint32 or Uint24 for a color composed of r,g,b, together without 'a' is 24bits(256=1b,256=1b,256=1b)=3b.

And then i somehow assign that pixel which is of 8bit at chosen location, an Uint24 or 32bit unsigned int.

How does this work?