Need help understanding repacks

[Anonymous0000]

Hi! I'm new to the forum, and I'm trying to understand the mechanism of repacking. Right now I'm using the Disk GUI for precompressing & decompress, but I'm trying to understand what each one does & what is the best option for compressing the file. The codes are pretty much auto generated so I guess coding isn't a problem for now.
I'm stuggling to find relevant information on this forum, as the information is pretty fragmented. Read through the xtool's docs but all it does is throwing a bunch of codes at me which does little explanation. I found almost no information on reddit or youtube that tells you what each compression method does or what the best one is for you; hence, I'm hoping someone could answer my questions:

For precompress:
1. Does it have to be the same compression logic as the game file?
2. Can I use multiple precompress parameter for the same game? When I did, there was an error code.
3. what's the difference between using xtool and xtool 2020? ie, what's the difference between using xzlib and zlib, or xlz4 vs lz4
4. Probably the most confusing question, what are the difference between using different precompress parameters? Or how do I select the right one? What are the pros/cons with each one, and has anyone done a tutorial instructing on selecting those based on your needs/hardware capability?
5. I've tried a couple of parameters on my game, but it yields suboptimal compression ratio than for example, fitgirl. How do I tweak parameters so that I can achieve even smaller size?
6. Is there a noob-friendly tutorial for ppl with 0 programming knowledge?

For installer:
1. How do I incorporate my setup.dll into the exe? or the autorun.inf?
2. I see that some repacks have limit ram usage option imbedded in their repacks, does it mean that I can select which type of decompressing method from the installer?

For decompress:
1. Can this be different from the precom parameter?
2. Can I have multiple parameters for decompress?
3. Where can I find detailed comparison for different paramters? Like for the same pc and same files, which decompress is faster & takes less resources?
4. I've tried ultra, high & normal for a particular decompress for the same file, but saw no noticeable difference, why?
5. Does parameter affect compression ratio?
6. What's the difference between LZMA and LZMA2 and XZ?
7. Dictionary size, is it bigger the better?

Most of my games are not on the sticky game index, and I'm trying to achieve a good compression ratio(like fitgirl size), I'd appreciate it if someone can answer my questions.