I am Level 70 (57) and I don’t know this game works

Hey guys, I picked this game up again after the Lord of Hatred expansion came out and I’m trying to get a few of the main concepts down. I watched a YouTube video on how to make a meta paladin build and all of my equipment is level 70 and legendary but I don’t know what else I should be doing. Been playing nonstop because I think it’s fun but I just don’t “get it”

The Occultist
- how do I know what aspects/codexes of power I should use? It feels very expensive to do this and I want to get it right
- Enchanting: when, why, how?

Jewelry
- what is rune crafting? That’s to add to the weapon sockets right?

Blacksmith
- master working or tempering? Which one should I focus more on?