Moodlets and Buffs

1. For the most part, yes. Buffs are called moodlets when they appear on your screen with all the colors and mood effects, like you are used to seeing. There are also hidden buffs.
2. Yes. You would apply a buff through loot, or basic_extras in an interaction, which a loot can be triggered by interactions.
3. Yes, I recommend starting with override mods first instead of jumping into custom files right away, that way you can play around with existing code to make yourself familiar. Scarlet's tutorials are a good place to do that for basics: https://simsvip.com/category/the-sims-4/how-to-create-a-mod/page/2/

Zer0's tutorials cover more intermediate stuff, like how to make custom buffs, loots, etc. Here is their directory, look at the top with the Start To Finish label: https://www.patreon.com/posts/35031545

Also, majority of mods are not scripts. The stuff in .packages and what you are about to do is called a tuning mod done with XML. Scripts in .ts4script use Python and are generally not needed until you get to advanced mods or mods that need injecting, and even then there are several tools out there that do the majority of injecting for you.