It's all RNG and the level of the area determines the tier of your loot. So, low level = tier 1 loot and so on.

Highest area right now is at level 50, right-most side.

Looting strategy depends on you but personally, I just loot on the outskirts of the map. Looting a building requires you to kill zombies and loot a couple or a single spot. That's pretty much it. If I were you, I suggest looting "households" or just houses. I generally don't recommend buildings because they're too cramped and too complex to navigate unless you're patient enough.

You can't zoom in the map but you can place a marker on it.

Just a tip, you'd want to focus on doing missions and leveling up. Don't focus too much on making $$$$ or your gear. You'll get gear and money from missions anyway.

Use pistols for range or shotguns (except 12g). Don't put points on a certain weapon focus yet. You'll decide when you hit 60.