Trying to wrap my head around preloaders

Hi There!
For some reason, I’m having a hard time understanding the logistics of the preloader. I’m not so much confused with the actual code, but where the code should exist in the movie–especially when other .swf files are loaded when the user interacts with a button.

Where do I put the preloader code? Do I attach it to a blank movie clip? Is it in the first frame of the first scene in the main timeline? Can multiple movie clips in one timeline act as a preloader for .swf files that will be loaded later on during user interaction? I’ve seen flash sites where multiple progress bars appear at the same time to load different content within the site. How is this done? Is it a good practice to place all content in a “hidden” scene only to ensure all elements are loaded–is this something people do at all?

Again, I understand the logic behind preloader code, but am unclear as to where to put the code in the movie, and if I have to make modifications to my completed movie beyond placing the code somewhere (i.e. making a hidden scene, stuffing all objects there, etc.)

I hope this makes sense. Any light that you can shed on this would be of great help.

With kind regards,
Laz