Extractinator can be found in random chests underground. It's used for turn Slush and Silt for gems, ores, etc.
If you are new, I suggest using Sticky Bomb. When you will get crimtane/demonite you can make pickaxe which can break ground.
You need Key Mold which is dropped by mobs after defeat Wall of Flesh and Plantera.
You need defeat WoF.
Yes, Hardmode adds more aggresive mobs. Also your hallow and crimson/corruption will expand faster.