Need Help With Open World Shooter

Okay so ive been working on my project and need some help still because im confused still after reading some information on here. I read that about level loading and level streaming, I just need a simple version. The game im creating is essentially like ARK except with modern weapons and more PVP based. So in my game you can build a base, find loot, help AI, fight other groups, and you also have to survive meaning you have to find food and water.

My question is, when creating a game like mine do you just have one big level with everything such as AI, loot, etc where you want it? or do you create smaller levels and combine them some how?

Secondly, How do you add the feature of being able to switch form 1st to 3rd person?

Lastly, how do you add the feature of were it saves your items to specific players? Do you worry about this when you make servers for the game?

I need a simple answer on how to make a open world shooter game like mine work. Please help. Thanks!