"Very good" means something is really, really good. It is "good" to a high degree ("very" is used as an intensifier). "Pretty good" and "fairly good" are the same. They both mean something is moderately "good" (the "goodness" is average).

They all mean pretty much the same thing. I think it just depends on how and why they are said

I would say "pretty good" is stronger than "rather good" which is stronger than "fairly good" - all degrees relative to very good which is the strongest of the above but not as strong as extremely good!

When I say your english is pretty god, I mean it's not bad as for you, or better to say I didn't expect you to speak so good. Very good I mean perfect, nothing to add A+/10

Those are all "empty" adverbs, don't have to worry and care about them.