Help a beginner

[TellASpray]

Hello, I wanna start repacks but I don't understand anything so here's some questions (I'm sorry if I ask questions that are frenquently asked)

1. What's the difference between compression and precompression ?
2. I've heard that the best compression protocol is not the same for every media, how can I know which protocol is better for each media (I know there are lists for games but I wanna know for those that are not listed)
3. How can I create and import highly customized skins/themes for setup installers
4. How can I compress as hell and make crazy setup like fitgirl.
5. Are there better softwares ?

If you have guides or explanation threads I'm interested.