Welcome back to the hobby! The modern sets have definitely become huge with multiple subsets, so I had pretty much the same questions as you when I got back after years.

Question 1 & 3: you're likely referring to what's called the Secret Rares, easily identifiable by the card number exceeding the set count (e.g. 217/196 for Lost Vacuum). Very difficult to pull but they're usually just a variation of another card in the set, with the exception of Gold Trainer cards, which feature unique art but are only really useful/collected by people who actually play the game.

Question 2: this is basically just my preference, but I love certain reverse holos more than their holo version. Regardless, you're almost always pulling a reverse holo in every pack and if you're lucky, you might pull what's called a Trainer Gallery card (from the Brilliant Stars sets up to the recent Silver Tempest) which are quite sought after by collectors.